s390x: pv: Fix diag318 PV fencing

Diag318 fencing needs to be determined on the current VM PV state and
not on the state that the VM has when we create the CPU model.

Fixes: fabdada9 ("s390: guest support for diagnose 0x318")
Reported-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Tested-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Collin Walling <walling@linux.ibm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Message-Id: <20201022103135.126033-3-frankja@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>