QAPISchema._make_features() takes a definition expression, and
extracts its 'features' member. The other ._make_FOO() leave
destructuring expressions to their callers. Change ._make_features()
to match them.
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200317115459.31821-17-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>