virtio-pmem: fixes and improvements

[ShadowCurse]

Changes

• Validate descriptors len field to be 4 since the code expects this
• Cache msync result for better efficiency in case multiple flush requests are presented at once
• Add rate-limiter support

Reason

Edge case handling and addition of missing features

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

❌ Patch coverage is 42.33577% with 79 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.88%. Comparing base (a6b64fc) to head (f525b6c).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/vmm/src/devices/virtio/pmem/device.rs	45.45%	36 Missing ⚠️
src/vmm/src/rpc_interface.rs	0.00%	18 Missing ⚠️
src/vmm/src/lib.rs	0.00%	12 Missing ⚠️
src/vmm/src/devices/virtio/pmem/event_handler.rs	0.00%	8 Missing ⚠️
src/firecracker/src/api_server/request/pmem.rs	78.26%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5789      +/-   ##
==========================================
- Coverage   83.06%   82.88%   -0.19%     
==========================================
  Files         276      276              
  Lines       29583    29708     +125     
==========================================
+ Hits        24573    24623      +50     
- Misses       5010     5085      +75     

Flag	Coverage Δ
5.10-m5n.metal	83.18% <42.33%> (-0.20%)	⬇️
5.10-m6a.metal	82.51% <42.33%> (-0.21%)	⬇️
5.10-m6g.metal	79.79% <42.33%> (-0.20%)	⬇️
5.10-m6i.metal	83.18% <42.33%> (-0.21%)	⬇️
5.10-m7a.metal-48xl	82.50% <42.33%> (-0.21%)	⬇️
5.10-m7g.metal	79.79% <42.33%> (-0.20%)	⬇️
5.10-m7i.metal-24xl	83.15% <42.33%> (-0.21%)	⬇️
5.10-m7i.metal-48xl	83.15% <42.33%> (-0.21%)	⬇️
5.10-m8g.metal-24xl	79.79% <42.33%> (-0.20%)	⬇️
5.10-m8g.metal-48xl	79.79% <42.33%> (-0.20%)	⬇️
5.10-m8i.metal-48xl	83.16% <42.33%> (-0.20%)	⬇️
5.10-m8i.metal-96xl	83.16% <42.33%> (-0.20%)	⬇️
6.1-m5n.metal	83.21% <42.33%> (-0.21%)	⬇️
6.1-m6a.metal	82.53% <42.33%> (-0.22%)	⬇️
6.1-m6g.metal	79.79% <42.33%> (-0.20%)	⬇️
6.1-m6i.metal	83.21% <42.33%> (-0.20%)	⬇️
6.1-m7a.metal-48xl	82.53% <42.33%> (-0.21%)	⬇️
6.1-m7g.metal	79.79% <42.33%> (-0.20%)	⬇️
6.1-m7i.metal-24xl	83.22% <42.33%> (-0.21%)	⬇️
6.1-m7i.metal-48xl	83.22% <42.33%> (-0.21%)	⬇️
6.1-m8g.metal-24xl	79.79% <42.33%> (-0.20%)	⬇️
6.1-m8g.metal-48xl	79.79% <42.33%> (-0.20%)	⬇️
6.1-m8i.metal-48xl	83.22% <42.33%> (-0.20%)	⬇️
6.1-m8i.metal-96xl	83.22% <42.33%> (-0.20%)	⬇️

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[JamesC1305]

Overall LGTM. The only thing I wonder is if it could do with integration tests for the rate limiting functionality, similar to how the block device has test_patch_drive_limiter.

[ShadowCurse]

Added rate-limiter integration tests as well

[Manciukic]

a few more nits for inconsistencies in unit test, error messages, and docs