qapi/introspect.py: add _gen_features helper

qapi/introspect.py: add _gen_features helper

_make_tree might receive a dict (a SchemaInfo object) or some other type
(usually, a string) for its obj parameter. Adding features information
should arguably be performed by the caller at such a time when we know
the type of the object and don't have to re-interrogate it.

Signed-off-by: John Snow <[email protected]>
Message-Id: <20210216021809[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>