vhost-user: Add MTU protocol feature and op

This patch implements VHOST_USER_PROTOCOL_F_NET_MTU
protocol feature and VHOST_USER_NET_SET_MTU request so
that the backend gets notified of the user defined host
MTU.

If backend supports VHOST_USER_PROTOCOL_F_REPLY_ACK,
QEMU assumes MTU is valid if success is returned.

Vhost-net driver sends this request through a new
vhost_net_set_mtu vhost_ops entry.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Aaron Conole <aconole@redhat.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
