So I'll leave this here for posterity... while the pre-built VM images don't seem to want to work, building my own image using Debian 11 is working great so far.
So, no problem with XCP-ng/Xen, just with the image built by the GNS3 team.
Thanks for the responses Oliver, much appreciated.
Wondering if anyone has had any luck getting the GNS3 VM to function under XCP-ng. I've tried both the vmware and hyper-v images and neither one wants to work, though each works flawlessly under player and hyper-v.
Google search has not been kind. The best I can come up with is something like "Xen has problems with nested virtualization," but nothing really all that helpful to get it working.
In short, any time I try to fire up QEMU, the intermediate VM crashes with no log entries or even a message from the kernel on the console.
Thanks,
-Troy