Packs and base64 encodes this :class:`Operation` as an XDR string.

def xdr(self):
        """Packs and base64 encodes this :class:`Operation` as an XDR string.

        """
        op = Xdr.StellarXDRPacker()
        op.pack_Operation(self.to_xdr_object())
        return base64.b64encode(op.get_buffer())