Same reasoning as commit 02a5c4c9
("qdev: Drop misleading qdev_free() function").  The qbus_free()
function removes the child from the namespace and decrements the
reference count.  It does not, however, guarantee to free the child
since the refcount may still be held.

Just call object_unparent() directly.
Suggested-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>