qapi: Generalize command policy checking

qapi: Generalize command policy checking

The code to check command policy can see special feature flag
'deprecated' as command flag QCO_DEPRECATED.  I want to make feature
flag 'unstable' visible there as well, so I can add policy for it.

To let me make it visible, add member @special_features (a bitset of
QapiSpecialFeature) to QmpCommand, and adjust the generator to pass it
through qmp_register_command().  Then replace "QCO_DEPRECATED in
@flags" by QAPI_DEPRECATED in @special_features", and drop
QCO_DEPRECATED.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Acked-by: John Snow <[email protected]>
Message-Id: <20211028102520[email protected]>
Reviewed-by: Juan Quintela <[email protected]>
Reviewed-by: Eric Blake <[email protected]>