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
@@ -2074,7 +2074,10 @@ void __init setup_IO_APIC(void)
         io_apic_irqs = ~PIC_IRQS;
 
     printk("ENABLING IO-APIC IRQs\n");
-    printk(" -> Using %s ACK method\n", ioapic_ack_new ? "new" : "old");
+    if (!directed_eoi_enabled && !ioapic_ack_forced) {
+        ioapic_ack_new = (nr_ioapics > 1);
+        printk(" -> Using %s ACK method\n", ioapic_ack_new ? "new" : "old");
+    }
 
     if (ioapic_ack_new) {
         ioapic_level_type.ack = irq_complete_move;