s390x/cpumodel: expose features and feature groups as properties

Let's add all features and feature groups as properties to all CPU models.
If the "host" CPU model is unknown, we can neither query nor change
features. KVM will just continue to work like it did until now.

We will not allow to enable features that were not part of the original
CPU model, because that could collide with the IBC in KVM.

Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Message-Id: <20160905085244.99980-11-dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
