Use u32 to describe vsock related buffer sizes

[roypat]

In #4556, we updated our generic virtio buffer handling code to describe buffer sizes using u32 instead of usize, since the virtio specification states that lengths of virtio-buffers fit into u32 (and header fields in the virtio protocol describing lengths are typed to be 32 bit integers).

From that work, a small follow up is possible in the vsock module, where we can now also describe lengths of vsock packages using u32 instead of usize. This will allow us to get rid of a few more explicit casts. Particularly, we can do the following changes to function signatures:

• VsockConnection::peer_avail_credit can return u32
• VsockPacket::buf_size can return u32
• VsockPacket::read_at_offset_from should be (&self, &mut T, u32, u32) -> Result<u32, VsockError> (e..g. the conversion to usize should only happen right around the write_volatile_at call
• ditto for VsockPacket::write_from_offset_to
  Maybe we can even get away with changing the arguments IoVecBuffer[Mut]::{read,write}_volatile_at to u32s

[brandonpike]

Can pick this one up as well as a followup to #4556.