This error can not happen normally. If it happens, it indicates
something very wrong, we should abort QEMU. Moreover, the
user can only refer to /machine/peripheral or /objects, not
/machine/unattached.
While at it, remove superfluous check about local_err.
Signed-off-by: Gonglei <arei.gonglei@huawei.com> Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com> Signed-off-by: Andreas Färber <afaerber@suse.de>