s390x/cpumodel: model PTFF subfunctions for Multiple-epoch facility

For now, the kernel does not properly indicate configured CPU subfunctions
to the guest, but simply uses the host values (as support in KVM is still
missing). That's why we missed to model the PTFF subfunctions that come
with Multiple-epoch facility.

Let's properly add these, along with a new feature group.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180205102935.14736-1-david@redhat.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>