image: Move image descriptors to own files

Move image object descriptors to own image-desc
file(s). This allow to reuse the code in other tools.
I had to move show declarations to cr-show.h as well.

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