vhost-net: vhost_migration_done is vhost-user specific

Either the callback is mandatory to implement, in which case an assert()
is more appropriate, or it's not and we can't tell much whether the
function should fail or not (given it's name, I guess it should silently
success by default). Instead, make the implementation mandatory and
vhost-user specific to be more clear about its usage.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/net/vhost_net.c

@@ -383,13 +383,11 @@ void vhost_net_cleanup(struct vhost_net *net)
 int vhost_net_notify_migration_done(struct vhost_net *net, char* mac_addr)
 {
     const VhostOps *vhost_ops = net->dev.vhost_ops;
-    int r = -1;
 
-    if (vhost_ops->vhost_migration_done) {
-        r = vhost_ops->vhost_migration_done(&net->dev, mac_addr);
-    }
+    assert(vhost_ops->backend_type == VHOST_BACKEND_TYPE_USER);
+    assert(vhost_ops->vhost_migration_done);
 
-    return r;
+    return vhost_ops->vhost_migration_done(&net->dev, mac_addr);
 }
 
 bool vhost_net_virtqueue_pending(VHostNetState *net, int idx)