xen/pciback: Expand the warning message to include domain id.

When a PCI device is transferred to another domain and it is still
in usage (from the internal perspective), mention which other
domain is using it to aid in debugging.

[v2: Truncate the verbose message per Jan Beulich suggestion]
[v3: Suggestions from Ian Campbell on the wording]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

drivers/xen/xen-pciback/xenbus.c

@@ -243,8 +243,8 @@ static int xen_pcibk_export_device(struct xen_pcibk_device *pdev,
 	dev_dbg(&dev->dev, "registering for %d\n", pdev->xdev->otherend_id);
 	if (xen_register_device_domain_owner(dev,
 					     pdev->xdev->otherend_id) != 0) {
-		dev_err(&dev->dev, "device has been assigned to another " \
-			"domain! Over-writting the ownership, but beware.\n");
+		dev_err(&dev->dev, "Stealing ownership from dom%d.\n",
+			xen_find_device_domain_owner(dev));
 		xen_unregister_device_domain_owner(dev);
 		xen_register_device_domain_owner(dev, pdev->xdev->otherend_id);
 	}