Fix nwfilter driver reload/shutdown handling when unprivileged

Fix nwfilter driver reload/shutdown handling when unprivileged

https://bugzilla.redhat.com/show_bug.cgi?id=903184

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
(cherry picked from commit abbec81bd0c9bf917f2c63045222734d7e4411fb)

Conflicts:
	src/nwfilter/nwfilter_driver.c - earlier changes f4ea67f and
79b8a56 related to using bool and auto-shutdown of drivers are not backported