I know you resolved this a different way but what I did when migrating P2V from a bare metal debian system to xcpng is using clonezilla I transferred the image via the network to the VM which was booted to clonezilla as well. Then I changed the boot option to UEFI in advanced, pressed escape on startup and then proceeded to add a boot menu entry for grub of the Debian system. I would assume the process would have been similar in your case as well to resolve.