virtio-balloon: fix virtio-balloon child refcount in transports

object_initialize() leaves the object with a refcount of 1.
object_property_add_child() adds its own reference which is dropped
again when the property is deleted.

The upshot of this is that we always have a refcount >= 1.  Upon hot
unplug the virtio-balloon child is not finalized!

Drop our reference after the child property has been added to the
parent.
Signed-off-by: NGonglei <arei.gonglei@huawei.com>
Reviewed-by: NCornelia Huck <cornelia.huck@de.ibm.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/s390x/virtio-ccw.c

@@ -900,7 +900,7 @@ static void virtio_ccw_balloon_instance_init(Object *obj)
     VirtIOBalloonCcw *dev = VIRTIO_BALLOON_CCW(obj);
     object_initialize(&dev->vdev, sizeof(dev->vdev), TYPE_VIRTIO_BALLOON);
     object_property_add_child(obj, "virtio-backend", OBJECT(&dev->vdev), NULL);
-
+    object_unref(OBJECT(&dev->vdev));
     object_property_add(obj, "guest-stats", "guest statistics",
                         balloon_ccw_stats_get_all, NULL, NULL, dev, NULL);
 

hw/virtio/virtio-pci.c

@@ -1325,7 +1325,7 @@ static void virtio_balloon_pci_instance_init(Object *obj)
     VirtIOBalloonPCI *dev = VIRTIO_BALLOON_PCI(obj);
     object_initialize(&dev->vdev, sizeof(dev->vdev), TYPE_VIRTIO_BALLOON);
     object_property_add_child(obj, "virtio-backend", OBJECT(&dev->vdev), NULL);
-
+    object_unref(OBJECT(&dev->vdev));
     object_property_add(obj, "guest-stats", "guest statistics",
                         balloon_pci_stats_get_all, NULL, NULL, dev,
                         NULL);