xenopsd's long-gone xenlight backend still referenced in many places

[ydirson]

The xenopsd architecture documentation still talks about the xenlight / libxl backend, whereas xenlight seems to be dead since at least 2018. It is even mentioned as recommended in places (eg. comment in xenopsd.ml).
The places where it appears would likely benefit from updated documentation.

[psafont]

I joined Citrix and I've never seen xenlight in the code, it should be removed from the docs :)