virtio-serial: Make sure virtqueue is ready before discarding data

This can happen if a port gets unplugged before guest has chance to
initialise vqs.
Reported-by: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NAmit Shah <amit.shah@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/virtio-serial-bus.c

@@ -117,6 +117,9 @@ static void discard_vq_data(VirtQueue *vq, VirtIODevice *vdev)
 {
     VirtQueueElement elem;
 
+    if (!virtio_queue_ready(vq)) {
+        return;
+    }
     while (virtqueue_pop(vq, &elem)) {
         virtqueue_push(vq, &elem, 0);
     }