vhost -q will require socket path
SET_VRING_NUM will setup the actual vring size
Add support for virtio_net_hdr
Update for v5 of QEMU patch

VhostUserMsg:
 - flags, lower 2 bits hold version (0x1 currently). bit 2 holds Reply bit
 - new field - size to hold the size of the following buffer (i.e. structures)

Add new message type VHOST_USER_ECHO. Handle it on the server side (send reply back).

vhost_user_send_fds - send only the requested message size
vhost_user_recv_fds - 2 step receive (hdr + buffer)

client/server - set proper message flags (version, reply bits)