This patch provides a way for virtio-net to notify the
backend about the host MTU set by the user.

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>