I have solved my issue. As you said, the jpa jars were not all on the class path. Very basic problem, and I have no idea why the error mentioned jboss-service.xml...
So, the solution:
Download jbpm-persistence-jpa-x.x.x.Final.jar and drools-persistence-jpa-x.x.x.Final.jar and add them to your class path! In my case, I had to add them to my .ear manually, and later on will have to add the new stuff on Maven.
Thank you for the help, Maciej :)