If you install the module (without changeing the standalone.xml) do you see that the module is started within the logfile?
What logmessages do you see if you add teh DS to standalone.xml?
I just experienced the same issue and while searching i got this forum thread.Yesterday i just fixed the above issue its because the jar files
Jar files path : D:\jboss-as-7.1.1.Final\modules\com\informix\main\
Extract the jar ifxjdbc.jar (using winrar) you will get below folders
Please open META-INF folder and add "services" folder inside that you just add a file named "java.sql.Driver" inside this just specify the driver class path like this "com.informix.jdbc.IfxDriver" and insert the same it to the jar file just drag the service folder to the jar file opened in the winrar
In the same way just add same file with class name "com.informix.jdbcx.IfxXADataSource"
After inserting the files in the both jars and deploy it once more then the driver will list.