page-xfer: Don't send zero pages if none found

When page-server gets a request for an absent pagemap from lazy-pages
daemon, it should not reply with "zero pages"'.

The pagemap should be completely in sync between src and dst and dst
should never request pages that are not present. Maybe we should return
-1 here? At least we'll have a chance that dump will unroll everything...

Acked-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>