The connections from the jboss XADataSourceLoader already have Autocommit= false. Are you using BMT? if so, why? What do you mean by "not sharable"? Transaction isolation has nothing to do with autocommit. If you don't set an autocommit value explicitly, the driver default (often read committed) is used.
In jboss at the moment you must get a connection after the beginning of a transaction and close it (really, return it to the pool) before the end of the transaction. With CMT, this means obtaining and closing the connection within each method call.