I never used mysql before, but by looking at this guide here
http://docs.jboss.org/jbpm/v5.4/userguide/ch.installer.html#d0e609
Isn't the <drivers> part should be like this:
<drivers>
<driver name="mysql" module="com.mysql">
<xa-datasource-class>com.mysql.jdbc.jdbc2.optional.MysqlXADataSource</xa-datasource-class>
</driver>
</drivers>
Regards,
Thomas Setiabudi