nodedev: Fix failing to parse PCI address for non-PCI network devices

nodedev: Fix failing to parse PCI address for non-PCI network devices

Commit 8708ca01c added virNetDevSwitchdevFeature() to check if a network
device has Switchdev capabilities. virNetDevSwitchdevFeature() attempts
to retrieve the PCI device associated with the network device, ignoring
non-PCI devices. It does so via the following call chain

  virNetDevSwitchdevFeature()->virNetDevGetPCIDevice()->
  virPCIGetDeviceAddressFromSysfsLink()

For non-PCI network devices (qeth, Xen vif, etc),
virPCIGetDeviceAddressFromSysfsLink() will report an error when
virPCIDeviceAddressParse() fails. virPCIDeviceAddressParse() also
logs an error. After commit 8708ca01c there are now two errors reported
for each non-PCI network device even though the errors are harmless.

To avoid the errors, introduce virNetDevIsPCIDevice() and use it in
virNetDevGetPCIDevice() before attempting to retrieve the associated
PCI device. virNetDevIsPCIDevice() uses the 'subsystem' property of the
device to determine if it is PCI. See the sysfs rules in kernel
documentation for more details

https://www.kernel.org/doc/html/latest/admin-guide/sysfs-rules.html