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 31.25000% with 88 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.80%. Comparing base (f314fdd) to head (24441ed).

Files with missing lines	Patch %	Lines
src/vmm/src/devices/virtio/pmem/device.rs	48.38%	32 Missing ⚠️
src/firecracker/src/api_server/request/pmem.rs	0.00%	23 Missing ⚠️
src/vmm/src/rpc_interface.rs	0.00%	13 Missing ⚠️
src/vmm/src/lib.rs	0.00%	12 Missing ⚠️
src/vmm/src/devices/virtio/pmem/event_handler.rs	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5789      +/-   ##
==========================================
- Coverage   83.02%   82.80%   -0.23%     
==========================================
  Files         276      276              
  Lines       29430    29547     +117     
==========================================
+ Hits        24435    24467      +32     
- Misses       4995     5080      +85     

Flag	Coverage Δ
5.10-m5n.metal	83.06% <31.25%> (-0.24%)	⬇️
5.10-m6a.metal	82.38% <31.25%> (-0.25%)	⬇️
5.10-m6g.metal	79.80% <31.25%> (-0.24%)	⬇️
5.10-m6i.metal	83.06% <31.25%> (-0.25%)	⬇️
5.10-m7a.metal-48xl	82.36% <31.25%> (-0.26%)	⬇️
5.10-m7g.metal	79.80% <31.25%> (-0.24%)	⬇️
5.10-m7i.metal-24xl	83.03% <31.25%> (-0.25%)	⬇️
5.10-m7i.metal-48xl	83.02% <31.25%> (-0.25%)	⬇️
5.10-m8g.metal-24xl	79.79% <31.25%> (-0.25%)	⬇️
5.10-m8g.metal-48xl	79.80% <31.25%> (-0.24%)	⬇️
5.10-m8i.metal-48xl	83.03% <31.25%> (-0.24%)	⬇️
5.10-m8i.metal-96xl	83.03% <31.25%> (-0.25%)	⬇️
6.1-m5n.metal	83.08% <31.25%> (-0.24%)	⬇️
6.1-m6a.metal	82.41% <31.25%> (-0.25%)	⬇️
6.1-m6g.metal	79.79% <31.25%> (-0.25%)	⬇️
6.1-m6i.metal	83.08% <31.25%> (-0.25%)	⬇️
6.1-m7a.metal-48xl	82.40% <31.25%> (-0.25%)	⬇️
6.1-m7g.metal	79.79% <31.25%> (-0.25%)	⬇️
6.1-m7i.metal-24xl	83.09% <31.25%> (-0.25%)	⬇️
6.1-m7i.metal-48xl	83.09% <31.25%> (-0.25%)	⬇️
6.1-m8g.metal-24xl	79.79% <31.25%> (-0.24%)	⬇️
6.1-m8g.metal-48xl	79.79% <31.25%> (-0.24%)	⬇️
6.1-m8i.metal-48xl	83.10% <31.25%> (-0.25%)	⬇️
6.1-m8i.metal-96xl	83.10% <31.25%> (-0.25%)	⬇️

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