MyRocks: add support for virtual columns

create table sbtest1 (pad varchar(60)) engine=rocksdb;

MariaDB [j2]> ALTER TABLE sbtest1 ADD reverse_pad char(60) GENERATED ALWAYS AS (reverse(pad)) VIRTUAL;

ERROR 1910 (HY000): ROCKSDB storage engine does not support generated columns

MySQL [test]> ALTER TABLE sbtest1 ADD reverse_pad char(60) GENERATED ALWAYS AS (reverse(pad)) VIRTUAL;

ERROR 3106 (HY000): 'Specified storage engine' is not supported for generated columns.