util: make virPCIGetNetName() more versatile

util: make virPCIGetNetName() more versatile

A single PCI device may have multiple netdevs associated with it. Each
of those netdevs will have a different phys_port_id entry in
sysfs. This patch modifies virPCIGetNetName() to allow selecting one
of the potential many netdevs in two different ways:

1) by setting the "idx" argument, the caller can select the 1st (0),
2nd (1), etc. netdev from the PCI device's net subdirectory.

2) If the physPortID arg is set (to a null-terminated string) then
virPCIGetNetName() returns the netdev that has that phys_port_id in
the sysfs file of the same name in the netdev's directory.