--enable-ovmf build time option was enabled in Debian xen packages as it does not require any non-free components, see this bug: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=858962 So, please, if possible, enable this option in Fedora xen packages.
I am considering your request, but you can actually use UEFI on the current Fedora 29 build when starting guests directly with xl. Assuming you have the edk2-ovmf package installed (which is likely) run something like cat /usr/share/OVMF/OVMF_{VARS,CODE}.fd >/var/tmp/ovmf.bin and set bios = 'ovmf' bios_path_override = '/var/tmp/ovmf.bin' serial = 'pty' in your domU guest configuration file in /etc/xen . I don't know how easy it is to do if you are doing it indirectly for example by using virt-manager.
xen-4.11.1-1.fc29 has been submitted as an update to Fedora 29. https://bodhi.fedoraproject.org/updates/FEDORA-2018-98e04c96dd
xen-4.11.1-1.fc29 has been pushed to the Fedora 29 testing repository. If problems still persist, please make note of it in this bug report. See https://fedoraproject.org/wiki/QA:Updates_Testing for instructions on how to install test updates. You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2018-98e04c96dd
xen-4.11.1-1.fc29 has been pushed to the Fedora 29 stable repository. If problems still persist, please make note of it in this bug report.