To update this, since this is the first result when you google "Windows 11 xcp-ng":

Development is on-going on the XAPI project, an minimal set of features is ready and will probably be available in XCP-ng 8.3 beta within a few weeks.

– @stormi on Sep 17.

I had to do the following to remove the reboot block:

xe vm-param-remove param-name=blocked-operations param-key=hard_reboot uuid=$UUID

And then:

xe vm-reboot force=true vm=$UUID