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>