Use absolute path for image files

Now I try to restore CWD and a relative path will be invalid.

Add new options -D to set image files directory.

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