criu: page-pipe: add ability to allocate IOVs

When collection of dumpee pages is followed by any parasite operation, we
cannot share iovs between parasite args area and page-pipe.
As a bonus we get some simplification of dump_one_shmem.

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