virtio-serial: Assert for virtio queue ready before virtqueue operations

In addition to the previous fix for calling do_flush_queued_data() only
when the virtqueue is ready, ensure do_flush_queued_data() gets a vq
that's suitably initialised.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

hw/virtio-serial-bus.c

@@ -117,6 +117,7 @@ static void do_flush_queued_data(VirtIOSerialPort *port, VirtQueue *vq,
     VirtQueueElement elem;
 
     assert(port || discard);
+    assert(virtio_queue_ready(vq));
 
     while ((discard || !port->throttled) && virtqueue_pop(vq, &elem)) {
         uint8_t *buf;