It's quite simple -- get parent's pagemap before dumping memory,
then collect _full_ pagemap of task putting holes into page-pipe
when the page is (soft-dirty-clear && in-parent-map). At the end
reset task's soft-dirty bits.

This requires kernel support from
http://comments.gmane.org/gmane.linux.kernel.mm/98283Signed-off-by: Pavel Emelyanov <xemul@parallels.com>