image: Open parent image dir with -at calls

After fixes with -W option we've changed the cwd at the
time parent images are opened. Use the -at syscall to
proerly access ones.

[ Cleanup and comment from xemul@ ]

Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>