xen/pciback: Fix "device has been assigned to X domain!" warning

The full warning is:
"pciback 0000:05:00.0: device has been assigned to 2 domain! Over-writting the ownership, but beware."

which is correct - the previous domain that was using the device
forgot to unregister the ownership. This patch fixes this by
calling the unregister ownership function when the PCI device is
relinquished from the guest domain.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>