Add missing spin_unlock_irq() at xen/arch/x86/irq.c

Changeset 9889:42a8e3101c6c reorganized the code on this file, and
missed this spin_unlock_irq().  Without this patch, my machine hangs
completely during boot.  With this, it works.

Signed-off-by: Juan Quintela <quintela@redhat.com> 
Signed-off-by: Chris Wright <chrisw@sous-sol.org>

xen/arch/x86/irq.c

@@ -318,6 +318,7 @@ static void __pirq_guest_eoi(struct domain *d, int irq)
     {
         ASSERT(cpus_empty(action->cpu_eoi_map));
         desc->handler->end(irq_to_vector(irq));
+        spin_unlock_irq(&desc->lock);
         return;
     }