I'm not even sure that stop_all_cpus() in xen_reboot() is all that
meaningful; if the guest tells Xen its rebooting, I'm pretty sure Xen
will sort out the other VCPUs.
I'll see if I can work out what the root-cause is here, but I suspect it
may be that irqs have been disabled at this point, and Xen IPIs map to
irqs and so are never delivered...