qdev: Use error_abort instead of using local_err

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>