A default install of Openfiler 2.3 has an issue when rebooted. On a default installation of Openfiler 2.3, clients cannot use iSCSI target disks after a reboot. In other words, it’ll work fine until you reboot it. This has been discussed at some length in Openfiler forums, and has an open bug filed against it.
Also note that it has nothing to do with using XenServer. The iscsi-target service will not properly restart after a reboot, period, whether you are using XenServer as a iSCSI client against the Openfiler target or not.
I added a section to my “Openfiler: The Missing Manual” that discusses what this looks like and how to fix it.