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

--- a/xen/arch/x86/io_apic.c
+++ b/xen/arch/x86/io_apic.c
@@ -1559,7 +1559,7 @@ static unsigned int startup_level_ioapic
     return 0; /* don't check for pending */
 }
 
-int __read_mostly ioapic_ack_new = 1;
+int __read_mostly ioapic_ack_new = -1;
 static void setup_ioapic_ack(char *s)
 {
     if ( !strcmp(s, "old") )
@@ -2062,6 +2062,8 @@ 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");