The setup instructions at https://dev.liferay.com/discover/deployment/-/knowledge_base/6-2/installing-liferay-on-jboss-7-1 may require a little modification or some sort of warning sentence.
Following them with mysql-connector-java-5.0.8-bin.jar will result into a:
Due to the below reason:
The workaround on mysql-connector-java-5.0.8-bin.jar I had to follow in order to have the portal starting is the same described here: https://developer.jboss.org/message/722269#722269
1) Create a file with name java.sql.driver within jboss-as-7.1.1.Final/modules/com/liferay/portal/main/mysql-connector-java-5.0.8-bin.jar\META-INF/services
2) Edit such file and add the text com.mysql.jdbc.Driver
mysql-connector-java-5.1.42-bin.jar will already contain the above file and the entry com.mysql.jdbc.Driver, but deleting the entry at line #2 com.mysql.fabric.jdbc.FabricMySQLDriver is required to have the portal starting