Make ipi_cpu() function as intended.

IPI's in Xen are implemented through hypervisor event channels.
The MP code creates a pair of IRQs for each base IPI per CPU
(one for IPI function dispatch calls, one for IPI bitmap dispatch calls.)
Using PCPU_GET() was returning the IRQ of the IPI handler for the
current CPU; thus calls to ipi_cpu() were sending itself a message.
Instead, looking up the IPI in the target CPU ipi-to-irq map is needed.

Note: This doesn't fix Xen SMP (far from it!) but it at least
sends IPI's to the right places. Next - sending IPIs..

PR:	135069

sys/xen/evtchn/evtchn.c

@@ -225,12 +225,15 @@ evtchn_do_upcall(struct trapframe *frame)
 	}
 }
 
+/*
+ * Send an IPI from the current CPU to the destination CPU.
+ */
 void
 ipi_pcpu(unsigned int cpu, int vector) 
 { 
         int irq;
 
-	irq = PCPU_GET(ipi_to_irq[vector]);
+	irq = pcpu_find(cpu)->pc_ipi_to_irq[vector];
 	
         notify_remote_via_irq(irq); 
 }