virtio-balloon: Check if balloon registration failed

Multiple balloon registrations are not allowed; check if the
registration with the qemu balloon api succeeded.  If not, fail the
device init.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio-balloon.c

@@ -269,6 +269,7 @@ static int virtio_balloon_load(QEMUFile *f, void *opaque, int version_id)
 VirtIODevice *virtio_balloon_init(DeviceState *dev)
 {
     VirtIOBalloon *s;
+    int ret;
 
     s = (VirtIOBalloon *)virtio_common_init("virtio-balloon",
                                             VIRTIO_ID_BALLOON,
@@ -278,12 +279,18 @@ VirtIODevice *virtio_balloon_init(DeviceState *dev)
     s->vdev.set_config = virtio_balloon_set_config;
     s->vdev.get_features = virtio_balloon_get_features;
 
+    ret = qemu_add_balloon_handler(virtio_balloon_to_target,
+                                   virtio_balloon_stat, s);
+    if (ret < 0) {
+        virtio_cleanup(&s->vdev);
+        return NULL;
+    }
+
     s->ivq = virtio_add_queue(&s->vdev, 128, virtio_balloon_handle_output);
     s->dvq = virtio_add_queue(&s->vdev, 128, virtio_balloon_handle_output);
     s->svq = virtio_add_queue(&s->vdev, 128, virtio_balloon_receive_stats);
 
     reset_stats(s);
-    qemu_add_balloon_handler(virtio_balloon_to_target, virtio_balloon_stat, s);
 
     register_savevm(dev, "virtio-balloon", -1, 1,
                     virtio_balloon_save, virtio_balloon_load, s);

hw/virtio-pci.c

@@ -788,6 +788,9 @@ static int virtio_balloon_init_pci(PCIDevice *pci_dev)
     VirtIODevice *vdev;
 
     vdev = virtio_balloon_init(&pci_dev->qdev);
+    if (!vdev) {
+        return -1;
+    }
     virtio_init_pci(proxy, vdev);
     return 0;
 }