xend: pci: find_parent: should return string rather than int

xend: pci: find_parent: should return string rather than int

Using changeset 19783: 61ec78692b13, device assignment can't work:
e.g., in find_the_uppermost_pci_bridge(),
 parent = dev_parent.find_parent()
...
 dev_parent = PciDevice(parent),
we can see parent['domain'] is int and in PciDevice's __init__,
int(dev['domain'], 16) would fail:
TypeError: int() can't convert non-string with explicit base

Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>