criu: pagemap: split read_pagemap_page into smaller chunks
criu: pagemap: split read_pagemap_page into smaller chunks

Introduce helper functions for reading pages from parent snapshot and for
reading pages from local pages*img

travis-ci: success for criu: pagemap: minor improvements
Signed-off-by: Mike Rapoport <6432ce95675be77d61ce23efe20f472ee21e9f31@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@virtuozzo.com>

eabatalov/criu,eabatalov/criu,eabatalov/criu,eabatalov/criu,eabatalov/criu,eabatalov/criu