s390x/ipl: support diagnose 308 subcodes 5 and 6

s390x/ipl: support diagnose 308 subcodes 5 and 6

To support dynamically updating the IPL device from inside the KVM
guest on the s390 platform, DIAG 308 instruction is intercepted
in QEMU to handle the request.

Subcode 5 allows to specify a new boot device, which is saved for
later in the s390_ipl device. This also allows to switch from an
external kernel to a boot device.

Subcode 6 retrieves boot device configuration that has been previously
set.

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>