virtio: check for vring setup in virtio_queue_update_used_idx

If the vring has not been set up, it is not necessary for vring_used_idx
to do anything (as is already the case when the caller is virtio_load).
This is harmless for now, but it will be a problem when the
MemoryRegionCache has not been set up.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio.c

@@ -2156,7 +2156,9 @@ void virtio_queue_set_last_avail_idx(VirtIODevice *vdev, int n, uint16_t idx)
 
 void virtio_queue_update_used_idx(VirtIODevice *vdev, int n)
 {
-    vdev->vq[n].used_idx = vring_used_idx(&vdev->vq[n]);
+    if (vdev->vq[n].vring.desc) {
+        vdev->vq[n].used_idx = vring_used_idx(&vdev->vq[n]);
+    }
 }
 
 void virtio_queue_invalidate_signalled_used(VirtIODevice *vdev, int n)