qapi: Don't use info as witness of implicit object type

A future patch will enable error reporting from the various
QAPISchema*.check() methods.  But to report an error related
to an implicit type, we'll need to associate a location with
the type (the same location as the top-level entity that is
causing the creation of the implicit type), and once we do
that, keying off of whether foo.info exists is no longer a
viable way to determine if foo is an implicit type.

Instead, add an is_implicit() method to QAPISchemaEntity, and use it.
It can be overridden later for ObjectType and EnumType, when implicit
instances of those classes gain info.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1444710158-8723-8-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi-visit.py

@@ -337,7 +337,8 @@ class QAPISchemaGenVisitVisitor(QAPISchemaVisitor):
 
     def visit_needed(self, entity):
         # Visit everything except implicit objects
-        return not isinstance(entity, QAPISchemaObjectType) or entity.info
+        return not (entity.is_implicit() and
+                    isinstance(entity, QAPISchemaObjectType))
 
     def visit_enum_type(self, name, info, values, prefix):
         self.decl += gen_visit_decl(name, scalar=True)