It's not that simple to get q35 working in Xen, however, it's in our TODO list for 2021. It's indeed not a priority for Xen project right now, because security issues are using 99% of main Xen devs.
No rush, I am not even expecting it to ever happen.
Xen works well as-is for server usage. And you know, maybe thats great too.
Kvm/Qemu for desktop, and Xen for servers.
Better devs focus on keeping Xen secure and updated, then spreading thin to add features most of Xen's userbase dont even need and devs cant keep updated.
On the other hand, GPGPU workloads is quickly gaining ground, so I actually believe this feature could be very important for Xen in the future.
Consumer grade GPUs are super cheap, and having good support within Xen could potentially make it a lot easier for startups and small company's to use GPUs on a larger scale.
I am afraid Kvm will "win" users if it keeps lagging behind Xen on features.
I guess we allready seen it happen(amazon aws comes to mind...).
I will still be choosing Xen wherever applicable
keep up the great and important work you all do, Xcp-ng has my love.
No need to convince me, that's why here at Vates, we already planned to work on q35.
I was just talking about biggest Xen contributors, who might have other priorities.