Use u32 to describe vsock related buffer sizes

[brandonpike]

Move to u32 for vsock module. We can translate from u32 to usize as needed.

Changes

• Change VsockConnection::peer_avail_credit to return u32
• Change VsockPacket::{buf_size, read_at_offset_from, write_from_offset_to} parameters & return types to u32
• Change IoVecBuffer[Mut]::{read,write}_volatile_at to u32

Closes https://github.com/firecracker-microvm/firecracker/issues/4627

Reason

Follow-up to https://github.com/firecracker-microvm/firecracker/pull/4556.

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.
• [x] 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.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.87500% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.08%. Comparing base (8f0dcb1) to head (d514899). Report is 302 commits behind head on main.

Files with missing lines	Patch %	Lines
src/vmm/src/devices/virtio/iovec.rs	94.73%	2 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4637   +/-   ##
=======================================
  Coverage   82.08%   82.08%           
=======================================
  Files         255      255           
  Lines       31257    31262    +5     
=======================================
+ Hits        25656    25661    +5     
  Misses       5601     5601           

Flag	Coverage Δ
4.14-c5n.metal	79.57% <96.87%> (+<0.01%)	:arrow_up:
4.14-c7g.metal	?
4.14-m5n.metal	79.56% <96.87%> (+<0.01%)	:arrow_up:
4.14-m6a.metal	78.78% <96.87%> (+<0.01%)	:arrow_up:
4.14-m6g.metal	76.60% <96.87%> (+<0.01%)	:arrow_up:
4.14-m6i.metal	79.55% <96.87%> (-0.01%)	:arrow_down:
4.14-m7g.metal	76.60% <96.87%> (+<0.01%)	:arrow_up:
5.10-c5n.metal	82.09% <96.87%> (+<0.01%)	:arrow_up:
5.10-c7g.metal	?
5.10-m5n.metal	82.08% <96.87%> (+<0.01%)	:arrow_up:
5.10-m6a.metal	81.38% <96.87%> (-0.01%)	:arrow_down:
5.10-m6g.metal	79.38% <96.87%> (+<0.01%)	:arrow_up:
5.10-m6i.metal	82.07% <96.87%> (-0.01%)	:arrow_down:
5.10-m7g.metal	79.38% <96.87%> (+<0.01%)	:arrow_up:
6.1-c5n.metal	82.08% <96.87%> (-0.01%)	:arrow_down:
6.1-c7g.metal	?
6.1-m5n.metal	82.08% <96.87%> (+0.01%)	:arrow_up:
6.1-m6a.metal	81.38% <96.87%> (+<0.01%)	:arrow_up:
6.1-m6g.metal	79.37% <96.87%> (-0.01%)	:arrow_down:
6.1-m6i.metal	82.07% <96.87%> (-0.01%)	:arrow_down:
6.1-m7g.metal	79.38% <96.87%> (+<0.01%)	:arrow_up:

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[roypat]

Subsumed by #4788