Change IoVecBuffer[Mut] len to u32

[brandonpike]

Changes

• Change IoVecBuffer[Mut] len to u32 (was usize)
• Add u32 overflow check (errors out rather than panics)

Closes https://github.com/firecracker-microvm/firecracker/issues/4548

Reason

Protect against overflows and use a consistent data type for virtio files.

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

• [x] If a specific issue led to this PR, this PR closes the issue.
• [x] The description of changes is clear and encompassing.
• [x] Any required documentation changes (code and docs) are included in this PR.
• [ ] API changes follow the Runbook for Firecracker API changes.
• [x] User-facing changes are mentioned in CHANGELOG.md.
• [x] All added/changed functionality is tested.
• [x] New TODOs link to an issue.
• [x] Commits meet contribution quality standards.

---

• [x] This functionality cannot be added in rust-vmm.

[brandonpike]

Fixing lint/build errors...

[brandonpike]

Local Tests on R7g.metal:

Checkstyle

./tools/devtool -y checkstyle
======================================================================= 13 passed in 22.86s =======================================================================
[Firecracker devtool 2024-04-24T22:10:26+00:00] Finished test run ...

Integration tests

./tools/devtool -y test -- ../tests/integration_tests/build/
======================================================= 8 passed, 1 skipped, 1 warning in 384.33s (0:06:24) =======================================================
[Firecracker devtool 2024-04-24T22:08:11+00:00] Finished test run ...

Warning:
/firecracker/tests/integration_tests/build/test_coverage.py:106: UserWarning: Not uploading coverage report due to missing CODECOV_TOKEN environment variable
    warnings.warn(

Performance Tests

./tools/devtool -y test --performance -c 1-10 -m 0 -- ../tests/integration_tests/ -m 'no_block_pr and not nonci' --log-cli-level=INFO
FAILED

Performance tests fail on R7g because of the graviton 3 processors having a different cpu register set?

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.29630% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 82.08%. Comparing base (0f39350) to head (ba36b81).

Files	Patch %	Lines
src/vmm/src/devices/virtio/vsock/mod.rs	0.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4556      +/-   ##
==========================================
- Coverage   82.08%   82.08%   -0.01%     
==========================================
  Files         255      255              
  Lines       31256    31258       +2     
==========================================
+ Hits        25658    25659       +1     
- Misses       5598     5599       +1     

Flag	Coverage Δ
4.14-c5n.metal	79.58% <96.29%> (-0.01%)	:arrow_down:
4.14-c7g.metal	?
4.14-m5n.metal	79.56% <96.29%> (-0.01%)	:arrow_down:
4.14-m6a.metal	78.79% <96.29%> (-0.01%)	:arrow_down:
4.14-m6g.metal	76.62% <96.29%> (-0.01%)	:arrow_down:
4.14-m6i.metal	79.55% <96.29%> (-0.01%)	:arrow_down:
4.14-m7g.metal	76.62% <96.29%> (-0.01%)	:arrow_down:
5.10-c5n.metal	82.09% <96.29%> (-0.01%)	:arrow_down:
5.10-c7g.metal	?
5.10-m5n.metal	82.08% <96.29%> (-0.01%)	:arrow_down:
5.10-m6a.metal	81.39% <96.29%> (+<0.01%)	:arrow_up:
5.10-m6g.metal	79.40% <96.29%> (-0.01%)	:arrow_down:
5.10-m6i.metal	82.07% <96.29%> (-0.01%)	:arrow_down:
5.10-m7g.metal	79.40% <96.29%> (-0.01%)	:arrow_down:
6.1-c5n.metal	82.09% <96.29%> (-0.01%)	:arrow_down:
6.1-c7g.metal	?
6.1-m5n.metal	82.08% <96.29%> (-0.01%)	:arrow_down:
6.1-m6a.metal	81.38% <96.29%> (-0.01%)	:arrow_down:
6.1-m6g.metal	79.40% <96.29%> (-0.01%)	:arrow_down:
6.1-m6i.metal	82.07% <96.29%> (-0.01%)	:arrow_down:
6.1-m7g.metal	79.40% <96.29%> (-0.01%)	:arrow_down:

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