s390x/kvm: Rework cmma management

Let's keep track of cmma enablement and move the mem_path check into
the actual enablement. This now also warns users that do not use
cpu-models about disabled cmma when using huge pages.

Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
