Commit a926f81d2f6c ("xen/cpuhotplug: Replace cpu_up/down() with
device_online/offline()") replaced cpu_down() with device_offline()
call which requires that the CPU has been registered before. This
registration, however, happens later from topology_init() which
is called as subsys_initcall(). setup_vcpu_hotplug_event(), on the
other hand, is invoked earlier, during arch_initcall().
As result, booting a PV(H) guest with vcpus < maxvcpus causes a crash.
Move setup_vcpu_hotplug_event() (and therefore setup_cpu_watcher()) to
late_initcall(). In addition, instead of performing all offlining steps
in setup_cpu_watcher() simply call disable_hotplug_cpu().
Fixes: a926f81d2f6c (xen/cpuhotplug: Replace cpu_up/down() with
device_online/offline()"
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
