During database recovery, a transaction with wsrep XID is
recovered from InnoDB in prepared state. However, when the
transaction is looked up with trx_get_trx_by_xid() in
innobase_commit_by_xid(), trx->xid gets cleared in
trx_get_trx_by_xid_low() and commit time serialization history
write does not update the wsrep XID in trx sys header for
that recovered trx. As a result the transaction gets
committed during recovery but the wsrep position does not
get updated appropriately.

As a fix, we preserve trx->xid for Galera over transaction
commit in recovery phase.

Fix authored by: Teemu Ollakka (GaleraCluster) and Marko Mäkelä.

	modified:   mysql-test/suite/galera/disabled.def
	modified:   mysql-test/suite/galera/r/galera_gcache_recover_full_gcache.result
	modified:   mysql-test/suite/galera/r/galera_gcache_recover_manytrx.result
	modified:   mysql-test/suite/galera/t/galera_gcache_recover_full_gcache.test
	modified:   mysql-test/suite/galera/t/galera_gcache_recover_manytrx.test
	modified:   storage/innobase/trx/trx0trx.cc
	modified:   storage/xtradb/trx/trx0trx.cc