Set a virtualchain field value.
        Used by implementations that generate extra consensus data at the end of a block

def set_virtualchain_field(cls, opdata, virtualchain_field, value):
        """
        Set a virtualchain field value.
        Used by implementations that generate extra consensus data at the end of a block
        """
        assert virtualchain_field in RESERVED_KEYS, 'Invalid field name {} (choose from {})'.format(virtualchain_field, ','.join(RESERVED_KEYS))
        opdata[virtualchain_field] = value