Inflight I/O: Fix the memory layout of queue region header

[stefano-garzarella]

From #88:

As defined in the vhost-user protocol[1], the last field of queue region
header should be a zero-sized array. However, current implementation uses
u64, which will cause a wrong inflight I/O memfd memory layout.

That PR was never merged, because we should fix the issue using vmm_sys_util::fam.

[amniik]

I will work on this and open a PR.