[feature/pcie] prevent unnecessary fmt

[Manciukic]

Changes

Use .ok_or_else when the Error needs to be computed by a function call, which may be expensive.
I fixed it in these places:

• MSI interrupt trigger
• clippy tracing
• cpu template helper

And I've added a clippy rule for the future.

Reason

I was looking at perf traces for a different thing and noticed an unexpected String format in a good path during interrupt trigger:

-    1.16%     0.29%  firecracker  firecracker        [.] <vmm::devices::virtio::transport::pci::device::VirtioInterruptMsix as vmm::devices::virtio::transport::VirtioInterrupt>::trigger                                                     ▒
   - 0.88% <vmm::devices::virtio::transport::pci::device::VirtioInterruptMsix as vmm::devices::virtio::transport::VirtioInterrupt>::trigger                                                                                                    ▒
      - <vmm::vstate::vm::MsiVectorGroup as vm_device::interrupt::InterruptSourceGroup>::trigger                                                                                                                                               ▒
         + 0.71% alloc::fmt::format::format_inner                                                                                                                                                                                              ▒
           0.05% enframe                                                                                                                                                                                                                       ▒
           0.05% __lock                                                                                                                                                                                                                        ▒
           0.01% __libc_malloc_impl                                                                                                                                                                                                            ▒
           0.01% size_to_class                                                                                                                                                                                                                 ▒
           0.00% get_stride                                                                                                                                                                                                                    ▒
           0.00% rdlock                                                                                                                                                                                                                        ▒
           0.00% core::ptr::drop_in_place<alloc::boxed::convert::<impl core::convert::From<alloc::string::String> for alloc::boxed::Box<dyn core::error::Error+core::marker::Sync+core::marker::Send>>::from::StringError>                     ▒
           0.00% default_malloc                                                                                                                                                                                                                ▒
           0.00% core::fmt::write                                                                                                                                                                                                              ▒
   + 0.28% libc_start_main_stage2                                                                                                                                                                                                              ▒
   + 0.00% 0x500007ffe                                                                                                                                                                                                                         ▒

This wastes 0.71% of the CPU consumed by Firecracker during the block io performance tests, which is very little, but the change is also a one-liner.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the Apache 2.0 license. For more information on following Developer
Certificate of Origin and signing off your commits, please check
CONTRIBUTING.md.

PR Checklist

• I have read and understand CONTRIBUTING.md.
• I have run tools/devtool checkbuild --all to verify that the PR passes
  build checks on all supported architectures.
• I have run tools/devtool checkstyle to verify that the PR passes the
  automated style checks.
• I have described what is done in these changes, why they are needed, and
  how they are solving the problem in a clear and encompassing way.
• I have updated any relevant documentation (both in code and in the docs)
  in the PR.
• I have mentioned all user-facing changes in CHANGELOG.md.
• If a specific issue led to this PR, this PR closes the issue.
• When making API changes, I have followed the
  Runbook for Firecracker API changes.
• I have tested all new and changed functionalities in unit tests and/or
  integration tests.
• I have linked an issue to every new TODO.

---

• This functionality cannot be added in rust-vmm.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.59%. Comparing base (1e675c2) to head (36cde07).
⚠️ Report is 2 commits behind head on feature/pcie.

Additional details and impacted files

@@              Coverage Diff              @@
##           feature/pcie    #5356   +/-   ##
=============================================
  Coverage         80.59%   80.59%           
=============================================
  Files               265      265           
  Lines             30690    30690           
=============================================
  Hits              24736    24736           
  Misses             5954     5954           

Flag	Coverage Δ
5.10-c5n.metal	80.38% <100.00%> (ø)
5.10-m5n.metal	80.37% <100.00%> (-0.01%)	⬇️
5.10-m6a.metal	79.59% <100.00%> (ø)
5.10-m6g.metal	76.89% <100.00%> (ø)
5.10-m6i.metal	80.36% <100.00%> (-0.01%)	⬇️
5.10-m7a.metal-48xl	79.57% <100.00%> (ø)
5.10-m7g.metal	76.89% <100.00%> (ø)
5.10-m7i.metal-24xl	80.34% <100.00%> (-0.01%)	⬇️
5.10-m7i.metal-48xl	80.34% <100.00%> (ø)
5.10-m8g.metal-24xl	76.88% <100.00%> (ø)
5.10-m8g.metal-48xl	76.88% <100.00%> (ø)
6.1-c5n.metal	80.42% <100.00%> (ø)
6.1-m5n.metal	80.42% <100.00%> (-0.01%)	⬇️
6.1-m6a.metal	79.63% <100.00%> (ø)
6.1-m6g.metal	76.89% <100.00%> (ø)
6.1-m6i.metal	80.41% <100.00%> (ø)
6.1-m7a.metal-48xl	79.61% <100.00%> (-0.01%)	⬇️
6.1-m7g.metal	76.89% <100.00%> (ø)
6.1-m7i.metal-24xl	80.43% <100.00%> (ø)
6.1-m7i.metal-48xl	80.43% <100.00%> (+<0.01%)	⬆️
6.1-m8g.metal-24xl	76.88% <100.00%> (ø)
6.1-m8g.metal-48xl	76.88% <100.00%> (ø)

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