pciback: rate limit error message from pciback_enable_msi()

... as being guest triggerable (e.g. by invoking XEN_PCI_OP_enable_msi
on a device not being MSI capable).

This is CVE-2013-0231 / XSA-43.

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/drivers/xen/pciback/conf_space_capability_msi.c
+++ b/drivers/xen/pciback/conf_space_capability_msi.c
@@ -11,13 +11,12 @@
 int pciback_enable_msi(struct pciback_device *pdev,
 		struct pci_dev *dev, struct xen_pci_op *op)
 {
-	int otherend = pdev->xdev->otherend_id;
-	int status;
-
-	status = pci_enable_msi(dev);
+	int status = pci_enable_msi(dev);
 
 	if (status) {
-		printk("error enable msi for guest %x status %x\n", otherend, status);
+		if (printk_ratelimit())
+			printk("error enabling MSI for guest %u status %d\n",
+			       pdev->xdev->otherend_id, status);
 		op->value = 0;
 		return XEN_PCI_ERR_op_failed;
 	}