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

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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.

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