utils_test/libvirt: Add a function pci_label_from_address.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
libvirt_xml/nodedev_xml: Add virt_functions in PCIXML class.

This patch add a support for virtual functions pci devices.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>