s390x/cpumodel: implement QMP interface "query-cpu-model-comparison"

Let's implement that interface by reusing our convertion code implemented
for expansion.

We use CPU generations and CPU features to calculate the result. This
means, that a zEC12 cannot simply be converted into a z13 by stripping
of features. This is required, as other magic values (e.g. maximum
address sizes) belong to a CPU generation and cannot simply be
emulated by an older generation.

Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Message-Id: <20160905085244.99980-30-dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>