libvirt_pci_passthrough: Rewrite the configuration file.

As the original way to test pci passthrough is not secure
for all host. This patch make user to provide the pci_label
to find the pci device.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Merge pull request #185 from yangdongsheng/libvirt_pci

libvirt_pci_passthrough: Rewrite the configuration file.