This one is a little bit more tricky that dump. On restore we
should open the chain of pagemap-s (by "parent" links). Then for
every pagemap with in_parent set we should go to parent and ask
for the page. Parent, in turn, should properly determine where
the respective page is in his pagemap and position the page.img's
position respectively.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>