s390/ipl: Fix boot order

The latest ipl code adaptions collided with some of the virtio
refactoring rework. This resulted in always booting the first
disk. Let's fix booting from a given ID.
The new code also checks for command lines without bootindex to
avoid random behaviour when accessing dev_st (==0).

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
