This patch allows advising guest with host MTU's by setting
host_mtu parameter.

If VIRTIO_NET_F_MTU has been successfully negotiated, MTU
value is passed to the backend.

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