nwfilter: don't crash listing filters in unprivileged daemon

nwfilter: don't crash listing filters in unprivileged daemon

The unprivileged libvirtd does not support nwfilter config, by leaves the
driver active. It is supposed to result in all APIs being an effective
no-op, but several APIs rely on driver->nwfilters being non-NULL, or they
will reference a NULL pointer. Rather than adding checks for NULL in many
places, just make sure  driver->nwfilters is always initialized.

Reviewed-by: John Ferlan <jferlan@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>