virtio-net: Allow qemu_announce_self to trigger virtio announcements

Expose the virtio-net self announcement capability and allow
qemu_announce_self() to call it.

These announces are caused by something external (i.e. the
announce-self command); they won't trigger if the migration
counter is triggering announces at the same time.

Signed-off-by: Vladislav Yasevich <vyasevic@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>