fix(virtqueue): make BufferElem::{len,capacity} return u32

[mkroening]

Closes #1514.

This was a usize before, which got cast to u32 without checks, which was wrong: 1749ab4
The fix for that restricted this to u16, though, which is too narrow.

u32 should be correct, though: https://docs.oasis-open.org/virtio/virtio/v1.2/cs01/virtio-v1.2-cs01.html#x1-430005