virtio: check for vring setup in virtio_queue_empty
If the vring has not been set up, there is nothing in the virtqueue.
virtio_queue_host_notifier_aio_poll calls virtio_queue_empty even in
this case; we have to filter it out just like virtio_queue_notify_aio_vq.
Reported-by: Gerd Hoffmann <kraxel@redhat.com> Tested-by: Alex Williamson <alex.williamson@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Tested-by: Laszlo Ersek <lersek@redhat.com> Tested-by: Cornelia Huck <cornelia.huck@de.ibm.com>