PT-202 pt-online-schema-change fails with virtual columns

Modified TableParser to ignore GENERATED columns from the columns list
used to construct SELECTs/INSERTs

t/pt-online-schema-change/samples/pt-202.sql

@@ -0,0 +1,9 @@
+CREATE SCHEMA IF NOT EXISTS test;
+USE test;
+DROP TABLE IF EXISTS t1;
+CREATE TABLE `test`.`t1` (
+`ID` int(11) NOT NULL,
+`Column2` int(11) DEFAULT NULL,
+`Column3` int(11) GENERATED ALWAYS AS ((`Column2` + 1)) STORED,
+PRIMARY KEY (`ID`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;