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

Let's provide a standardized interface to compare two CPU models.
"query-cpu-model-compare" takes two models and returns  how they compare
in a specific configuration.

The result will give guarantees about runnability. E.g. if a CPU model A
is a subset of CPU model B, model A is guaranteed to run in configurations
where model B runs, but not the other way around (might or might not run).

Usually, CPU features or CPU generations are used to calculate the result.
If a model is not guaranteed to run in a certain environment (e.g.
incompatible), a  compatible one can be created by "baselining" both models
(follow up patch).

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: <20160905085244.99980-27-dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>