qapi/introspect.py: use _make_tree for features nodes

At present, we open-code this in _make_tree itself; but if the structure
of the tree changes, this is brittle. Use an explicit recursive call to
_make_tree when appropriate to help keep the interior node typing
consistent.

A consequence of doing this is that the 'ifcond' key of the features
dict will be omitted when ifcond is false-ish, just like it is omitted
in top-level calls to _make_tree. This also increases consistency in our
handling of this property.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20210216021809.134886-4-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/introspect.py

@@ -30,7 +30,9 @@ def _make_tree(obj, ifcond, features, extra=None):
     if ifcond:
         extra['if'] = ifcond
     if features:
-        obj['features'] = [(f.name, {'if': f.ifcond}) for f in features]
+        obj['features'] = [
+            _make_tree(f.name, f.ifcond, None) for f in features
+        ]
     if extra:
         return (obj, extra)
     return obj