Change default IO-APIC ack mode for single IO-APIC systems to old-style.

Index: xen-4.2.0-testing/xen/arch/x86/io_apic.c
===================================================================
--- xen-4.2.0-testing.orig/xen/arch/x86/io_apic.c
+++ xen-4.2.0-testing/xen/arch/x86/io_apic.c
@@ -43,7 +43,7 @@ static struct { int pin, apic; } ioapic_
 static DEFINE_SPINLOCK(ioapic_lock);
 
 bool_t __read_mostly skip_ioapic_setup;
-bool_t __read_mostly ioapic_ack_new = 1;
+bool_t __read_mostly ioapic_ack_new = -1;
 bool_t __read_mostly ioapic_ack_forced = 0;
 
 #ifndef sis_apic_bug
@@ -2012,6 +2012,8 @@ void __init setup_IO_APIC(void)
     else
         io_apic_irqs = ~PIC_IRQS;
 
+    if (ioapic_ack_new < 0)
+        ioapic_ack_new = (nr_ioapics > 1);
     printk("ENABLING IO-APIC IRQs\n");
     printk(" -> Using %s ACK method\n", ioapic_ack_new ? "new" : "old");