So I got a weird one. I have installed the latest XCP 8.3 updates and rebooted my server. All the VM's I had on my test server worked perfectly fine except for a single Debian 9 VM that would start booting and then "power off" just as the kernel started to spit stuff on the display.
I banged around with it for a while and what I found is its a kernel crash somewhere when SMP is initialized. If I only give a single vProc to the VM, it boots normally and all works fine.
At this point, its not causing me any more problems because I jsut rebuilt the VM on something more modern (Rocky 9)
I have perserved this VM if the dev's would like to get more debugging information or wish to try anything. I can also capture the logs if its of interest.