Return the XML value of the enumeration value *enum_val*.

def to_xml(cls, enum_val):
        """
        Return the XML value of the enumeration value *enum_val*.
        """
        if enum_val not in cls._member_to_xml:
            raise ValueError(
                "value '%s' not in enumeration %s" % (enum_val, cls.__name__)
            )
        return cls._member_to_xml[enum_val]