python: treat flags as default argument with value 0

The following four functions have not changed because default arguments
have to come after positional arguments. Changing them will break the
the binding APIs.

migrate(self, dconn, flags, dname, uri, bandwidth):
migrate2(self, dconn, dxml, flags, dname, uri, bandwidth):
migrateToURI(self, duri, flags, dname, bandwidth):
migrateToURI2(self, dconnuri, miguri, dxml, flags, dname, bandwidth):

generator.py

@@ -1489,6 +1489,8 @@ def buildWrappers(module):
                     if n != index:
                         classes.write(", %s" % arg[0])
                     n = n + 1
+                if arg[0] == "flags":
+                    classes.write("=0");
                 classes.write("):\n")
                 writeDoc(module, name, args, '        ', classes)
                 n = 0