xen: Fix find_unbound_irq in presence of ioapic irqs.

Don't break the assumption that the first 16 irqs are ISA irqs;
make sure that the irq is actually free before using it.

Use dynamic_irq_init_keep_chip_data instead of
dynamic_irq_init so that chip_data is not NULL (a NULL chip_data breaks
setup_vector_irq).

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

drivers/xen/events.c

@@ -340,9 +340,18 @@ static int find_unbound_irq(void)
 	int irq;
 	struct irq_desc *desc;
 
-	for (irq = 0; irq < nr_irqs; irq++)
+	for (irq = 0; irq < nr_irqs; irq++) {
+		desc = irq_to_desc(irq);
+		/* only 0->15 have init'd desc; handle irq > 16 */
+		if (desc == NULL)
+			break;
+		if (desc->chip == &no_irq_chip)
+			break;
+		if (desc->chip != &xen_dynamic_chip)
+			continue;
 		if (irq_info[irq].type == IRQT_UNBOUND)
 			break;
+	}
 
 	if (irq == nr_irqs)
 		panic("No available IRQ to bind to: increase nr_irqs!\n");
@@ -351,7 +360,7 @@ static int find_unbound_irq(void)
 	if (WARN_ON(desc == NULL))
 		return -1;
 
-	dynamic_irq_init(irq);
+	dynamic_irq_init_keep_chip_data(irq);
 
 	return irq;
 }