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-rng 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>