dump: Move pagemap parent checks from mem-dump to page-xfer

When writing pagemaps to page-server we want both page-*.img and pagemap-*.img
to be on remote host. But the subsequent pre-dump/dump with parent images will
try to access pagemap-*img-s to check for hole dumped being present in it.

To handle this, move checks for hole being backed by something in parent into
page-local-xfer. In case of page-server dump it will be page-server who will
check the holes when writing them.

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