Reuse the IoVecBuffer on TX

[JackThomson2]

Changes

Removes the allocation of the IoVecBuffer in the virtio net device during tx by reusing the same instance for each message.

To do this we have to implement Send on the type for it to work, I have tried to cover the unsafe sections the best I can.

Reason

Reduce the number of allocation

Ticket here: #4549

License Acceptance

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PR Checklist

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

---

• This functionality cannot be added in rust-vmm.

[bchalios]

Just a couple of comments regarding unnecessary SAFETY comments. Other than that LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.99%. Comparing base (be2f41a) to head (97858a1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4589      +/-   ##
==========================================
+ Coverage   81.97%   81.99%   +0.01%     
==========================================
  Files         254      254              
  Lines       31169    31193      +24     
==========================================
+ Hits        25552    25576      +24     
  Misses       5617     5617              

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5.10-m6g.metal	79.03% <100.00%> (+0.01%)	⬆️
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6.1-m5n.metal	81.99% <100.00%> (+0.01%)	⬆️
6.1-m6a.metal	81.29% <100.00%> (+0.01%)	⬆️
6.1-m6g.metal	79.03% <100.00%> (+0.01%)	⬆️
6.1-m6i.metal	81.99% <100.00%> (+0.02%)	⬆️
6.1-m7g.metal	79.03% <100.00%> (+0.01%)	⬆️

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[roypat]

Looks pretty good! Could you just have a look at my two comments and the test failures in CI? They're related to clippy complaining about missing safety comments. For the ones in mod test, you can just add a #![allow(clippy::undocumented_unsafe_blocks)] at the top of the module. No point in annotating these in unittests.

[roypat]

🎉

[bchalios]