qapi: Generalize enum member policy checking

qapi: Generalize enum member policy checking

The code to check enumeration value policy can see special feature
flag 'deprecated' in QEnumLookup member flags[value].  I want to make
feature flag 'unstable' visible there as well, so I can add policy for
it.

Instead of extending flags[], replace it by @special_features (a
bitset of QapiSpecialFeature), because that's how special features get
passed around elsewhere.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Acked-by: John Snow <jsnow@redhat.com>
Message-Id: <20211028102520.747396-8-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>