s390x/cpumodel: ignore csske for expansion

s390x/cpumodel: ignore csske for expansion

csske will be removed in a future machine. Ignore it for expanding the
cpu model. Otherwise qemu falls back to z9.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: qemu-stable@nongnu.org
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20190429090250.7648-3-borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
(cherry picked from commit eaf6f642abf1d4d24791b70728d4068428fc4658)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>