KVM: s390: we are always in czam mode

Independent of the underlying hardware, kvm will now always handle
SIGP SET ARCHITECTURE as if czam were enabled. Therefore, let's not
only forward that bit but always set it.

While at it, add a comment regarding STHYI.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170829143108.14703-1-david@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>