Richard, thanks for your perseverence. Can you please test the patch I have just sent to the bug ?
You should be able to patch <0001-init-scripts-Do-not-kill-per-domain-qemu-processes.patch /etc/init.d/xen It WFM. That is, I checked that it kills the global qemu but leaves a domU one alone. I am considering shipping this change in a stable update for Xen 4.8. Ian.