Make sriov_config finish its work before starting NetworkManager

The SR-IOV config stub installed as a systemd service sometimes continue to run even when NetworkManager is already starting, which is causing connections to flap or fail, especially when using the link_mode: switchdev option (which causes all PFs and VFs to disappear and reappear).

This is easily solved by adding NetworkManager.service as a dependency of sriov_config in /etc/systemd/system/sriov_config.service:

[Unit]
Before=NetworkManager.service