xend: pci: only extract the exact pci BDFs

xend: pci: only extract the exact pci BDFs

On some hosts:
[root@localhost ~]# ls /sys/bus/pci/devices/0000:00:05.0/
0000:00:05.0:pcie00  0000:05:00.0          class   driver  local_cpus
resource          subsystem_vendor
0000:00:05.0:pcie01  broken_parity_status  config  enable  modalias
subsystem         uevent
0000:00:05.0:pcie02  bus                   device  irq     power
subsystem_device  vendor

Here we should only get 0000:05:00.0, but we also get 0000:00:05.0
unexpectedly. With this patch, xend only extracts the exact BDF(s).

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