Don't use separate bind mount for container

Since we're not not mounting anything but the base filesystem outside
the container we no longer need the separate bind mount at
/var/lib/docker/container/$id/root in order to see the base filesystem
without extra mounts. So, we drop this and mount (again) the container
root directly at the real basefs mountpoint.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)

integration/utils_test.go

@@ -71,7 +71,7 @@ func containerFileExists(eng *engine.Engine, id, dir string, t utils.Fataler) bo
 		t.Fatal(err)
 	}
 	defer c.Unmount()
-	if _, err := os.Stat(path.Join(c.BasefsPath(), dir)); err != nil {
+	if _, err := os.Stat(path.Join(c.RootfsPath(), dir)); err != nil {
 		if os.IsNotExist(err) {
 			return false
 		}