Inflight I/O: Implement missing traits

This commit implements the get and set inflight fd members of the
VhostUserSlaveReqHandlerMut trait, which is used to pass inflight
I/O queue tracking regions as memfds.

Fixes #14.

Signed-off-by: Harshavardhan Unnibhavi <harshanavkis@gmail.com>

Author: harshanavkis