Use --root instead of --aufs-root

When dumping Docker containers using the AUFS graph driver, we can
use the --root option instead of --aufs-root for specifying the
container's root.  This patch obviates the need for --aufs-root
and makes dump CLI more consistent with restore CLI.

Signed-off-by: Saied Kazemi <saied@google.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>