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

Let's implement that interface by reusing our conversion code and
lookup code for CPU definitions.

In order to find a compatible CPU model, we first detect the maximum
possible CPU generation and then try to find a maximum model, satisfying
all base features (not exceeding the maximum generation).

Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Message-Id: <20160905085244.99980-31-dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>