crit: Files now sit in another image

When merging files into one image we've forgotten about crit x
feature that scans image files by names.

https://github.com/xemul/criu/issues/381



The patch was made for master (as in criu-dev there was problem
with pstree_item.pid parsing), but should apply to dev smoothly.

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