Expose QSqlDriverPrivate dbmsType in public QSqlDriver api

dbmsType was previously kept as a private variable in QSqlDriverPrivate,
however it's particularly useful for QODBC users.

[ChangeLog][QtSql][QSqlDriver] Add support for determining DBMS type from SQL driver.

Change-Id: If1c221520da9ac4ccef85a02db078679d76eac92
Reviewed-by: Mark Brand <mabrand@mabrand.nl>