Don't probe network drivers when initializing SR-IOV VFS

The sriov-config stub provided by os-net-config takes forever to start when using a large number of VFs in SR-IOV mode. Kernels 4.12+ provided a sysfs tunable to avoid driver probing for the VFs and it would be helpful to be able to configure this tunable from the os-net-config sriov_pf config.

The sysfs tunable is /sys/class/net/PF/device/sriov_drivers_autoprobe, available since 4.12+.

This really speeds up startup time, especially when also using link_mode: switchdev, since all VFs started and probed are deleted then recreated as representor types when switching to the switchdev mode.

I fixed my own installation by adding the corresponding sysfs write in os_net_config/sriov_config.py:188:

                sriov_drivers_autoprobe = os.path.join(_SYS_CLASS_NET, item['name'],
                                                       "device/sriov_drivers_autoprobe")
                with open(sriov_drivers_autoprobe, 'w') as a:
                    a.write("0")