KVM: s390: add debug logging for cpu model subfunctions

As userspace can now get/set the subfunctions we want to trace those.
This will allow to also check QEMUs cpu model vs. what the real
hardware provides.

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