qapi: Use predicate callback to determine visit filtering

Previously, qapi-types and qapi-visit filtered out implicit
objects during visit_object_type() by using 'info' (works since
implicit objects do not [yet] have associated info); meanwhile
qapi-introspect filtered out all schema types on the first pass
by returning a python type from visit_begin(), which was then
used at a distance in QAPISchema.visit() to do the filtering.

Rather than keeping these ad hoc approaches, add a new visitor
callback visit_needed() which returns False to skip a given
entity, and which defaults to True unless overridden.  Use the
new mechanism to simplify all three filtering visitors.

No change to the generated code.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1444710158-8723-2-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
diff --git a/scripts/qapi-introspect.py b/scripts/qapi-introspect.py
index 7d39320..c0dad66 100644
--- a/scripts/qapi-introspect.py
+++ b/scripts/qapi-introspect.py
@@ -54,7 +54,6 @@
         self._jsons = []
         self._used_types = []
         self._name_map = {}
-        return QAPISchemaType   # don't visit types for now
 
     def visit_end(self):
         # visit the types that are actually used
@@ -82,6 +81,10 @@
         self._used_types = None
         self._name_map = None
 
+    def visit_needed(self, entity):
+        # Ignore types on first pass; visit_end() will pick up used types
+        return not isinstance(entity, QAPISchemaType)
+
     def _name(self, name):
         if self._unmask:
             return name