show: don't show RAW images in show_all

If an image isn't specified, crtool shows content of all images.
Now crtools aborts in this case due to BUG_ON(1) in show methods
for raw images. I sugget to skip such images.

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