s390x/kvm: provide stfle.81

stfle.81 (ppa15) is a transparent facility that can be passed to the
guest without the need to implement hypervisor support. As this feature
can be provided by firmware we add it to all full models.

Cc: qemu-stable@nongnu.org
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <20180118085628.40798-4-borntraeger@de.ibm.com>
Reviewed-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>