s390/ipl: fix ipl with -no-reboot

s390/ipl: fix ipl with -no-reboot

kexec/kdump as well as the bootloader use a subcode of diagnose 308
that is supposed to reset the I/O subsystem but not comprise a full
"reboot". With the latest refactoring this is now broken when
-no-reboot is used or when libvirt acts on a reboot QMP event, for
example a virt-install from iso images.

We need to mark these "subsystem resets" as special.

Fixes: a30fb811cbe9 (s390x: refactor reset/reipl handling)
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180622102928.173420-1-borntraeger@de.ibm.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>