Your persistence.xml should probably look like this (with the changes to make it work for MySQL of course):
https://github.com/krisv/jbpm/blob/master/jbpm-installer/db/persistence.xml
Regarding the query, you should also have orm.xml in your META-INF directory in your classpath:
https://github.com/krisv/jbpm/blob/master/jbpm-persistence-jpa/src/test/resources/META-INF/orm.xml
Hope that helps.
Kris