util: Add phys_port_name support on virPCIGetNetName

virPCIGetNetName is used to get the name of the netdev associated with
a particular PCI device. This is used when we have a VF name, but need
the PF name in order to send a netlink command (e.g. in order to
get/set the MAC address of the VF).

In simple cases there is a single netdev associated with any PCI
device, so it is easy to figure out the PF netdev for a VF - just look
for the PCI device that has the VF listed in its "virtfns" directory;
the only name in the "net" subdirectory of that PCI device's sysfs
directory is the PF netdev that is upstream of the VF in question.

In some cases there can be more than one netdev in a PCI device's net
directory though. In the past, the only case of this was for SR-IOV
NICs that could have multiple PF's per PCI device. In this case, all
PF netdevs associated with a PCI address would be listed in the "net"
subdirectory of the PCI device's directory in sysfs. At the same time,
all VF netdevs and all PF netdevs have a phys_port_id in their sysfs,
so the way to learn the correct PF netdev for a particular VF netdev
is to search through the list of devices in the net subdirectory of
the PF's PCI device, looking for the one netdev with a "phys_port_id"
matching that of the VF netdev.

But starting in kernel 5.8, the NVIDIA Mellanox driver began linking
the VFs' representor netdevs to the PF PCI address [1], and so the VF
representor netdevs would also show up in the net
subdirectory. However, all of the devices that do so also only have a
single PF netdev for any given PCI address.

This means that the net directory of the PCI device can still hold
multiple net devices, but only one of them will be the PF netdev (the
others are VF representors):

$ ls '/sys/bus/pci/devices/0000:82:00.0/net'
ens1f0  eth0  eth1

In this case the way to find the PF device is to look at the
"phys_port_name" attribute of each netdev in sysfs. All PF devices
have a phys_port_name matching a particular regex

  (p[0-9]+$)|(p[0-9]+s[0-9]+$)

Since there can only be one PF in the entire list of devices, once we
match that regex, we've found the PF netdev.

[1] - https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git/


      commit/?id=123f0f53dd64b67e34142485fe866a8a581f12f1

Co-Authored-by: Moshe Levi <moshele@nvidia.com>
Signed-off-by: Dmytro Linkin <dlinkin@nvidia.com>
Reviewed-by: Adrian Chiris <adrianc@nvidia.com>
Reviewed-by: Laine Stump <laine@redhat.com>