img: Introduce v1.1 images (v2)

These images have common magic in front of per-image one. With
this we have 3 "types" of images -- inventory (head), other
images, service files. The latter would be stats (not an image,
just happen to be in PB format) and irmap cache (not an image
again, just auxiliary thing which is in PB for convenience).

Since inventory file is the first one we read on restore it's
OK to set the global "new images" flag there. Dump (write) is
always in new format.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Ruslan Kuprieiev <rkuprieiev@cloudlinux.com>
Acked-by: Andrew Vagin <avagin@odin.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>