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

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

[Move path variable and g_free() call inside if statement scope as
suggested by Michael
-- Stefan]

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