x86: Assert in_irq() while processing guest-bound interrupts.

Actually a fair amount of infrastructure is onvolved these days, and
we'd like subsystems we call into know that we're in IRQ context so
they can handle that or BUG/ASSERT if appropriate.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>

xen/arch/x86/irq.c

@@ -63,7 +63,9 @@ asmlinkage void do_IRQ(struct cpu_user_regs *regs)
 
     if ( likely(desc->status & IRQ_GUEST) )
     {
+        irq_enter();
         __do_IRQ_guest(vector);
+        irq_exit();
         spin_unlock(&desc->lock);
         return;
     }