xen/pciback: Don't setup an fake IRQ handler for SR-IOV devices.

If we try to setup an fake IRQ handler for legacy interrupts
for devices that only have MSI-X (most if not all SR-IOV cards),
we will fail with this:

pciback[0000:01:10.0]: failed to install fake IRQ handler for IRQ 0! (rc:-38)

Since those cards don't have anything in dev->irq.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
diff --git a/drivers/xen/xen-pciback/pciback_ops.c b/drivers/xen/xen-pciback/pciback_ops.c
index 6c398fd..28a2a55 100644
--- a/drivers/xen/xen-pciback/pciback_ops.c
+++ b/drivers/xen/xen-pciback/pciback_ops.c
@@ -48,6 +48,13 @@
 	if (enable)
 		dev_data->irq = dev->irq;
 
+	/*
+	 * SR-IOV devices in all use MSI-X and have no legacy
+	 * interrupts, so inhibit creating a fake IRQ handler for them.
+	 */
+	if (dev_data->irq == 0)
+		goto out;
+
 	dev_dbg(&dev->dev, "%s: #%d %s %s%s %s-> %s\n",
 		dev_data->irq_name,
 		dev_data->irq,