virtio-balloon: discard virtqueue element on reset

The one pending element is being freed but not discarded on device
reset, which causes svq->inuse to creep up, eventually hitting the
"Virtqueue size exceeded" error.

Properly discarding the element on device reset makes sure that its
buffers are unmapped and the inuse counter stays balanced.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Roman Kagan <rkagan@virtuozzo.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NLadi Prosek <lprosek@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 104e70ca)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

hw/virtio/virtio-balloon.c

@@ -478,6 +478,7 @@ static void virtio_balloon_device_reset(VirtIODevice *vdev)
     VirtIOBalloon *s = VIRTIO_BALLOON(vdev);
 
     if (s->stats_vq_elem != NULL) {
+        virtqueue_discard(s->svq, s->stats_vq_elem, 0);
         g_free(s->stats_vq_elem);
         s->stats_vq_elem = NULL;
     }