qdev: Use NULL instead of local_err for qbus_child unrealize

Forcefully unrealize all children regardless of errors in earlier
iterations (if any). We should keep going with cleanup operation
rather than report an error immediately. Therefore store the first
child unrealization failure and propagate it at the end. We also
forcefully unregister vmsd and unrealize actual object, too.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Andreas Färber <afaerber@suse.de>
