python: treat flags as default argument with value 0

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):