vhost: add assert() to check runtime behaviour

All these functions must be called only after the backend is connected.
They are called from virtio-net.c, after either virtio or link status
change.

The check for nc->peer->link_down should ensure vhost_net_{start,stop}()
are always called between vhost_user_{start,stop}().
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/virtio/vhost.c

@@ -1242,6 +1242,9 @@ void vhost_virtqueue_mask(struct vhost_dev *hdev, VirtIODevice *vdev, int n,
     int r, index = n - hdev->vq_index;
     struct vhost_vring_file file;
 
+    /* should only be called after backend is connected */
+    assert(hdev->vhost_ops);
+
     if (mask) {
         assert(vdev->use_guest_notifier_mask);
         file.fd = event_notifier_get_fd(&hdev->vqs[index].masked_notifier);
@@ -1288,6 +1291,9 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
 {
     int i, r;
 
+    /* should only be called after backend is connected */
+    assert(hdev->vhost_ops);
+
     hdev->started = true;
 
     r = vhost_dev_set_features(hdev, hdev->log_enabled);
@@ -1350,6 +1356,9 @@ void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev)
 {
     int i;
 
+    /* should only be called after backend is connected */
+    assert(hdev->vhost_ops);
+
     for (i = 0; i < hdev->nvqs; ++i) {
         vhost_virtqueue_stop(hdev,
                              vdev,