Boot from SAN depends on SAN-based disk arrays with either hardware Fibre Channel or HBA iSCSI adapter support on the host. For a fully redundant boot from SAN environment, you must configure multiple paths for I/O access. To do so, ensure that the root device has multipath support enabled. For information about whether multipath is available for your SAN environment, consult your storage vendor or administrator. If you have multiple paths available, you can enable multipathing in your Citrix Hypervisor deployment upon installation.

XenServer 7.x adds Software-boot-from-iSCSI for Cisco UCS
Yes for XenServer 6.1 and later (XenServer 5.6 SP1 added support for boot from SAN with multi-pathing support for Fibre Channel and iSCSI HBAs)

Note: Rolling Pool Upgrade should not be used with Boot from SAN environments. For more information on upgrading boot from SAN environments see Appendix B of the XenServer 7.5 Installation Guide: http://bit.ly/2sK4wGn