qapi: Split machine-target.json off target.json and misc.json

qapi: Split machine-target.json off target.json and misc.json

Move commands query-cpu-definitions, query-cpu-model-baseline,
query-cpu-model-comparison, and query-cpu-model-expansion with their
types from target.json to machine-target.json.  Also move types
CpuModelInfo, CpuModelExpansionType, and CpuModelCompareResult from
misc.json there.  Add machine-target.json to MAINTAINERS section
"Machine core".

Cc: Eduardo Habkost <[email protected]>
Cc: Marcel Apfelbaum <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>
Message-Id: <20190619201050[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>
[Commit message typo fixed]