qmp: add QMP interface "query-cpu-model-baseline"

Let's provide a standardized interface to baseline two CPU models, to
create a third, compatible one. This is especially helpful when two
CPU models are not identical, but a CPU model is required that is
guaranteed to run under both configurations, where the original models run.

"query-cpu-model-baseline" takes two CPU models and returns a third,
compatible model. The result will always be a static CPU model.

Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Message-Id: <20160905085244.99980-28-dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>