img: Introduce the notion of a raw image

Raw images are supposed to be files with non-crtools contents.
E.g. tarballs with tmpfs contents or iptables-save output. Another
example is -- ip tool addr and route dumps we're about to merge
soon.

Having these type of images allows to use the open_image and the
cr_fdset engine easily.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>