restore: add fast path to find a parent pstree item

Taking into account the way the dump saves pstrees in the image.

If pstree.img isn't edited, a slow path should not be executed at all.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>