KVM: s390: protvirt: handle secure guest prefix pages

The SPX instruction is handled by the ultravisor. We do get a
notification intercept, though. Let us update our internal view.

In addition to that, when the guest prefix page is not secure, an
intercept 112 (0x70) is indicated. Let us make the prefix pages
secure again.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
[borntraeger@de.ibm.com: patch merging, splitting, fixing]
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>