tests/vhost-user-bridge: propose GUEST_ANNOUNCE feature

The backend has to know whether VIRTIO_NET_F_GUEST_ANNOUNCE was
negotiated, so, as a hack we propose the feature by
vhost-user-bridge during the feature negotiation.

Signed-off-by: Victor Kaplansky <victork@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
