Disable NWFilter driver completely when unprivileged

Running libvirtd unprivileged results in a warning message from
the NWFilter driver

  virNWFilterSnoopLeaseFileRefresh:1882 : open("/var/run/libvirt/network/nwfilter.ltmp"): No such file or directory

Since it requires privileged network access, this driver should
not even run when unprivileged.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

src/nwfilter/nwfilter_driver.c

@@ -68,6 +68,9 @@ static int
 nwfilterDriverStartup(int privileged) {
     char *base = NULL;
 
+    if (!privileged)
+        return 0;
+
     if (virNWFilterIPAddrMapInit() < 0)
         return -1;
     if (virNWFilterLearnInit() < 0)