s390/cpumodel: remove KSS from the default model of z14

The SIE_KSS feature will allow a guest to use KSS for a nested guest.
To create a nested guest the SIE_F2 facility is still necessary.
Since SIE_F2 is not part of the default model it does not make
a lot of sense to provide the SIE_KSS feature in the default model.
Let's also create a dependency check.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Jason J. Herne <jjherne@linux.vnet.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Message-Id: <1500550051-7821-2-git-send-email-borntraeger@de.ibm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
