virtio-net: fix the memory leak in rxfilter_notify()

object_get_canonical_path() returns a gchar*, it should be freed by the
caller.

Signed-off-by: Amos Kong <akong@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Vlad Yasevich <vyasevic@redhat.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>