xen/evtchn: clear secondary CPUs' cpu_evtchn_mask[] after restore

xen/evtchn: clear secondary CPUs' cpu_evtchn_mask[] after restore

To bind all event channels to CPU#0, it is not sufficient to set all
of its cpu_evtchn_mask[] bits; all other CPUs also need to get their
bits cleared. Otherwise, evtchn_do_upcall() will start handling
interrupts on CPUs they're not intended to run on, which can be
particularly bad for per-CPU ones.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Keir Fraser <keir@xen.org>