cr: Preparations for snapshot

1. Directory with images may have a "parent" symlink pointing to the
   place where the previous snapshot is
2. Each pagemap will have "in_parent" bit, which means, that the
   pages for this pagemap entry are not in the respective page.img
   but in parent
3. New --leave-running option to use with --snapshot not to kill
   tasks after snapshot

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