virtio: validate the existence of handle_output before calling it

We don't validate the existence of handle_output which may let a buggy
guest to trigger a SIGSEV easily. E.g:

1) write 10 to queue_sel to a virtio net device with only 1 queue
2) setup an arbitrary pfn
3) then notify queue 10

Fixing this by validating the existence of handle_output before.

Cc: qemu-stable@nongnu.org
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: NJason Wang <jasowang@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NDon Koch <dkoch@verizon.com>
Reviewed-by: NFam Zheng <famz@redhat.com>

hw/virtio/virtio.c

@@ -759,8 +759,9 @@ void virtio_queue_set_align(VirtIODevice *vdev, int n, int align)
 
 void virtio_queue_notify_vq(VirtQueue *vq)
 {
-    if (vq->vring.desc) {
+    if (vq->vring.desc && vq->handle_output) {
         VirtIODevice *vdev = vq->vdev;
+
         trace_virtio_queue_notify(vdev, vq - vdev->vq, vq);
         vq->handle_output(vdev, vq);
     }