fix: use temporary buffer for loading vmstate file

[roypat]

In commit e7504ae ("refactor: cleanup vmm::snapshot module"),
firecracker started reading the snapshot vmstate file in a single pass
instead of first loading it into a Vec and then deserializing. This
seems to have caused some performance regression due to the deserializer
doing many successive reads, resulting in many read(2) syscalls.
Fix this by going back to first reading the snapshot file into a buffer,
and then deserializing from slice instead.

Signed-off-by: Patrick Roy [email protected]## Changes

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Reason

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License Acceptance

By submitting this pull request, I confirm that my contribution is made under
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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.
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---

• This functionality cannot be added in rust-vmm.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.41%. Comparing base (57ea0a3) to head (5d9c4ba).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5394      +/-   ##
==========================================
+ Coverage   82.36%   82.41%   +0.05%     
==========================================
  Files         266      266              
  Lines       30571    30580       +9     
==========================================
+ Hits        25180    25203      +23     
+ Misses       5391     5377      -14     

Flag	Coverage Δ
5.10-c5n.metal	82.39% <100.00%> (+0.01%)	⬆️
5.10-m5n.metal	82.39% <100.00%> (+0.01%)	⬆️
5.10-m6a.metal	81.67% <100.00%> (+0.01%)	⬆️
5.10-m6g.metal	78.99% <100.00%> (+<0.01%)	⬆️
5.10-m6i.metal	82.38% <100.00%> (+0.02%)	⬆️
5.10-m7a.metal-48xl	81.65% <100.00%> (?)
5.10-m7g.metal	78.99% <100.00%> (+<0.01%)	⬆️
5.10-m7i.metal-24xl	82.36% <100.00%> (?)
5.10-m7i.metal-48xl	82.35% <100.00%> (?)
5.10-m8g.metal-24xl	78.99% <100.00%> (?)
5.10-m8g.metal-48xl	78.99% <100.00%> (?)
6.1-c5n.metal	82.43% <100.00%> (+0.01%)	⬆️
6.1-m5n.metal	82.43% <100.00%> (+0.02%)	⬆️
6.1-m6a.metal	81.71% <100.00%> (+0.02%)	⬆️
6.1-m6g.metal	78.99% <100.00%> (+<0.01%)	⬆️
6.1-m6i.metal	82.42% <100.00%> (+0.02%)	⬆️
6.1-m7a.metal-48xl	81.70% <100.00%> (?)
6.1-m7g.metal	78.99% <100.00%> (+<0.01%)	⬆️
6.1-m7i.metal-24xl	82.44% <100.00%> (?)
6.1-m7i.metal-48xl	82.44% <100.00%> (?)
6.1-m8g.metal-24xl	78.99% <100.00%> (?)
6.1-m8g.metal-48xl	78.99% <100.00%> (?)

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