Gets values of 'column' in 'record' in 'table'.

        This method is corresponding to the following ovs-vsctl command::

            $ ovs-vsctl get TBL REC COL

def db_get_val(self, table, record, column):
        """
        Gets values of 'column' in 'record' in 'table'.

        This method is corresponding to the following ovs-vsctl command::

            $ ovs-vsctl get TBL REC COL
        """
        command = ovs_vsctl.VSCtlCommand('get', (table, record, column))
        self.run_command([command])
        assert len(command.result) == 1
        return command.result[0]