Since now we drain pages out of parasite, we can invent any format for
page dumps. Let is be ... prorobuf one! :)

Another thing to keep in mind, is that we're about to use splices and
implement iterative migration, so it's better to have actual pages be
page-aligned in the image.

And -- backward compatibility. That said the new format is:

1. pagemap-... file which contains a header (currently with a ID of
   the image with pages, see below) and an array of <nr_pages:vaddr>
   pairs. The first value means "how many pages to take from the
   file with pages (see below)" and the second -- where in the task
   address space to put them. Simple.

2. pages-... file which containes only pages one by one (thus aligned
   as we want).

This patch breaks backward compatibility (old images with pages wil
be restored and then crash). Need to do it before v0.5 release.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>