Pull up following revision(s) (requested by bouyer in ticket #1668):

sys/arch/xen/xen/xen_clock.c: revision 1.10
sys/arch/xen/xen/xen_clock.c: revision 1.12
(applied to sys/arch/xen/xen/clock.c)

Unmask event after VCPUOP_stop_periodic_timer and
initializing ci->ci_xen_hardclock_systime_ns, to avoid a possible race with
xen_timer_handler()

Unmask event after arming the one-shot timer in clock initialisation,
to avoid a possible race with xen_timer_handler() updating
ci_xen_hardclock_systime_ns while we're reading it.

Pointed out by Taylor R Campbell

(martin)