KVM: s390: add debug tracing for cpu features of CPU model

The cpu model already traces the cpu facilities, the ibc and
guest CPU ids. We should do the same for the cpu features (on
success only).

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