XenServer 7.0 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 7.0 Installation Guide: http://bit.ly/2d8BkSV