fwiw, at the lists.xen.org thread mentioned above, using the ovmf-enabled xen pkgs from homde:olh:xen-4.6, and systemd-boot as a Guest bootloader (booting Arch, which has a modern systemd version), I can boot Xen Guest via UEFI using Xen bios=ovmf. I'll run similar tests when the changes make into official 'Virtualization' repo pkgs.