Elimination of unneeded generics and lots of .map_err()

[roypat]

A collection of cleanups I accumulated on a local branch while prototyping for #4522. Mainly

• Clean up error types in builder.rs slightly to avoid a lot of unsightly .map_err calls
• Clean up a lot of unnecessary generics in fdt.rs
• Add a constant to make our left shifts by 20 less mysterious.

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 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

Attention: Patch coverage is 96.22642% with 2 lines in your changes missing coverage. Please review.

Project coverage is 83.19%. Comparing base (6b5f4b1) to head (52104ba).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
src/vmm/src/builder.rs	94.73%	1 Missing ⚠️
src/vmm/src/vstate/memory.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5106      +/-   ##
==========================================
+ Coverage   83.13%   83.19%   +0.06%     
==========================================
  Files         249      249              
  Lines       26937    26885      -52     
==========================================
- Hits        22393    22368      -25     
+ Misses       4544     4517      -27     

Flag	Coverage Δ
5.10-c5n.metal	83.60% <90.90%> (+0.09%)	⬆️
5.10-m5n.metal	83.60% <90.90%> (+0.09%)	⬆️
5.10-m6a.metal	82.80% <90.90%> (+0.10%)	⬆️
5.10-m6g.metal	79.60% <90.56%> (+0.03%)	⬆️
5.10-m6i.metal	83.60% <90.90%> (+0.09%)	⬆️
5.10-m7a.metal-48xl	82.80% <90.90%> (?)
5.10-m7g.metal	79.60% <90.56%> (+0.03%)	⬆️
6.1-c5n.metal	83.65% <90.90%> (+0.09%)	⬆️
6.1-m5n.metal	83.65% <90.90%> (+0.09%)	⬆️
6.1-m6a.metal	82.84% <90.90%> (+0.09%)	⬆️
6.1-m6g.metal	79.60% <90.56%> (+0.04%)	⬆️
6.1-m6i.metal	83.64% <90.90%> (+0.09%)	⬆️
6.1-m7a.metal-48xl	82.84% <90.90%> (?)
6.1-m7g.metal	79.60% <90.56%> (+0.03%)	⬆️

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