xen: fix memory leak in __xen_pcibk_add_pci_dev()

It need to free dev_entry when it failed to assign to a new
slot on the virtual PCI bus.

smatch says:
 drivers/xen/xen-pciback/vpci.c:142 __xen_pcibk_add_pci_dev() warn:
possible memory leak of 'dev_entry'

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

drivers/xen/xen-pciback/vpci.c

@@ -137,6 +137,8 @@ unlock:
 	/* Publish this device. */
 	if (!err)
 		err = publish_cb(pdev, 0, 0, PCI_DEVFN(slot, func), devid);
+	else
+		kfree(dev_entry);
 
 out:
 	return err;