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

In order to expand CPU models, we create temporary cpus that handle the
feature/group parsing. Only CPU feature properties are expanded.

When converting the data structure back, we always fall back to the
static base CPU model, which is by definition migration-safe.

Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Message-Id: <20160905085244.99980-29-dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>