Currently we store the images version in the core file. This is
bad, since core file describes a single process (or thread) and
says nothing about the images set as a whole (let alone the fact
that it's being parsed too late).

Thus introduce the inventory image file which describes the image
set the way we need (want). For now the only entry in it is the
images version. In the future it can be extended.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>