target-ppc: Report CPU aliases for QMP

The QMP query-cpu-definitions implementation iterated over CPU classes
only, which were getting less and less as aliases were extracted.

Keep them in QMP as valid -cpu arguments even if not guaranteed stable.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
