xen/pci: Check for PCI bridge before using it.

Some SR-IOV devices may use more than one bus number, but there is no real bridges
because that have internal routing mechanism. So need to check whether the bridge is
existing before using it.

Signed-off-by: Yang Zhang <yang.z.zhang@Intel.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>