Ok, but changing to the bitronix for tomcat7, what exception are you getting now?
Because re reading this thread again I found that the original exception was:
org.jboss.resteasy.spi.UnhandledException: java.lang.RuntimeException: Could not initialize stateful knowledge session: No Persistence provider for EntityManager named org.jbpm.persistence.jpa
And that's because inside your persistence XML file you define the org.jbpm.task persistence unit.
Cheers