s390x/ipl: drop reipl parameters on resets

Whenever a reboot initiated by the guest is done, the reipl parameters should
remain valid. The disk configured by the guest is to be used for
ipl'ing. External reboot/reset request (e.g. via virsh reset guest) should
completely reset the guest to the initial state, and therefore also reset the
reipl parameters, resulting in an ipl behaviour of the initially configured
guest. This could be an external kernel or a disk.

Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Signed-off-by: Fan Zhang <zhangfan@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
