I got the example for a book and converted to Mariadb
the code is :

#include <QCoreApplication>
#include <QtSql/QSql>
#include <QtSql/QSqlDatabase>
#include <QtSql/QSqlQuery>
#include <QDebug>

int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);

QSqlDatabase db = QSqlDatabase::addDatabase("test");
db.setHostName("192.168.X.X");
db.setPort(3306);
db.setDatabaseName(("test"));
db.setUserName("----");
db.setPassword("----");
if(db.open())
{
    qDebug() << "Connesso";
}
else
{
    qDebug() << "Fallita connessione";
    return 0;
}

return a.exec();

}

QTPLUGIN += qsqlmysql

This is not needed.
You need to use 'QMYSQL' as database driver - please show us how you try to open the database.