KVM: s390: implement CPU model only facilities

KVM: s390: implement CPU model only facilities

CVE-2017-5715 (Spectre v2 s390x)

Some facilities should only be provided to the guest, if they are
enabled by a CPU model. This allows us to avoid capabilities and
to simply fall back to the cpumodel for deciding about a facility
without enabling it for older QEMUs or QEMUs without a CPU
model.

Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
(cherry picked from commit c3b9e3e1ea1c1d1524b56b6734711db2a6fc2163)
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Acked-by: Kamal Mostafa <kamal@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>