avoid crash on HVM domain destroy with PCI passthrough

c/s bac6334b5 "move domain to cpupool0 before destroying it" introduced a
problem when destroying a HVM domain with PCI passthrough enabled. The
moving of the domain to cpupool0 includes moving the pirqs to the cpupool0
cpus, but the event channel infrastructure already is unusable for the
domain. So just avoid moving pirqs for dying domains.

Signed-off-by: Juergen Gross <jgross@suse.com>

xen/common/schedule.c

@@ -297,7 +297,8 @@ int sched_move_domain(struct domain *d, struct cpupool *c)
         spin_unlock_irq(lock);
 
         v->sched_priv = vcpu_priv[v->vcpu_id];
-        evtchn_move_pirqs(v);
+        if ( !d->is_dying )
+            evtchn_move_pirqs(v);
 
         new_p = cpumask_cycle(new_p, c->cpu_valid);