s390x: protvirt: Set guest IPL PSW

s390x: protvirt: Set guest IPL PSW

Handling of CPU reset and setting of the IPL psw from guest storage at
offset 0 is done by a Ultravisor call. Let's only fetch it if
necessary.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20200319131921.2367-11-frankja@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>