Fix nwfilter driver reload/shutdown handling when unprivileged

Fix nwfilter driver reload/shutdown handling when unprivileged

Although the nwfilter driver skips startup when running in a
session libvirtd, it did not skip reload or shutdown. This
caused errors to be reported when sending SIGHUP to libvirtd,
and caused an abort() in libdbus on shutdown due to trying
to remove a dbus filter that was never added