@john-c
I feel your pain, however, the main difference between VMware support in Ansible and XenServer/XCP-ng is that VMware has a whole working group with a dozen of regular members and contributors:
https://github.com/ansible/community/wiki/VMware
Major contributors are all Red Hat or VMware employees i.e. people paid to do it. There is no such thing for XenServer/XCP-ng. Citrix never showed any interest in supporting Ansible. Netscaler is the only Citrix product that has a decent Ansible support.
To help you better understand how Ansible as a project works, here are some points from my personal adventure:
- To be able to contribute new modules to Ansible or any of the official collections, you need to implement extensive unit and integration tests. I understand the requirement. Ansible/Red Hat wants to maintain a high level of quality and to easily (and in automated way) detect any regressions. That's all good but implementing tests is harder and more work than implementing modules themselves. What's very very helpful in case of VMware is that there is a whole simulator called
govcsim developed by VMware. You can test your modules against the simulator with ease and automate all the tests with little effort. To my knowledge, there is no simulator available for XenAPI. If such simulator does exist, it is most likely kept in secret by Citrix. If Citrix was ever to release this simulator, that would be a HUGE step forward.
- If you want to contribute new modules to Ansible or any of the official collections, someone has to review your code. Not many people are willing to do so and have the power to include your code to Ansible. As a matter of fact, finding reviewers and begging them for help is the hardest thing of all. I had some tremendous luck to acquire the interest of Abhijeet Kasurde, one of the top Ansible guys, to review my code and to eventually include
xenserver_guest_* modules into Ansible. The guy handles VMware in Ansible... surprise! 
My xenserver_guest module was included without any unit or integration tests but for other modules I had to implement them. Luckily, they were simple and I had a luck to find a reviewer for tests also. When I wanted to upgrade xenserver_guest module with new functionality, they required unit and integration tests. I eventually implemented tests for xenserver_guest module but it was a huge undertaking and the amount of code involved easily dwarfed the module itself. I basically ended up implementing a barebone XenAPI simulator. This is where I hit a road block. No one, even the people that initially supported me, wanted to review this monstrosity of test+simulator. It was never included in Ansible.
- If you don't want to rely on external reviewers then you have to form a team, or if possible, a work group. That way you can review each others code and include it in Ansible without external support. Everything is pretty much handled by bots. If you gain a high enough status in Ansible project, you could get permissions to merge the code yourself without relying on anyone, not even bots. Should I mention that I failed to ever find any good Python programmer that is into Ansible and interested enough to form a team with me?
- You can skip all this struggle if you just maintain you own collection of modules but then you cannot rely on existing Ansible tooling that will do all the testing, linting, sanity checks, spell checks and such. You are on your own.
After a lot of struggle I eventually lost any interest as I was wasting a lot of time and life had to go on. Not much people showed interest in xenserver_guest_* Ansible modules either. My employer also ditched XenServer/XCP-ng in favor of VMware a few years back. Even with all the Broadcom/VMware situation, we got a super good deal with Broadcom because of our deployment size and commitment so we are sticking with VMWare.
All in all, if Ansible support for XenServer/XCP-ng and Xen Orchestra on par with VMware support is ever to see the light of day, these prerequisites are required:
- Publicly available XenAPI simulator is a must
- A working group of at least three people with knowledge in Python, Ansible and XenAPI committed to the cause
- Possibly corporate and financial backing by Citrix, Vates? or some other third party
Having any official Ansible support for XenServer/XCP-ng was (and is) a miracle to this day. A miracle I was blessed with and a huge learning experience for me.
Sorry for the long post. It is not my intention to discourage people but I think everyone should understand why XenServer/XCP-ng does not enjoy better Ansible support. There is much much more to it than just having a willingness to do anything.
. All of my modules have been merged upstream. I've updated the first post with new info.
. If you have your templates in XVA format and want to have them accessible to all your pool members, you have to put them on a shared SR. They will still be per pool. I don't do not know of a way to have them centralized so that they are available cross pool.