page-pipe: add compatible iovec

struct iovec may have different size for dumpee.
But that reason, pages dump will fail (with added debug to pie):
(00.011440) page-pipe: Page pipe:
(00.011441) page-pipe: * 1 pipes 8/523 iovs:
(00.011442) page-pipe: 	buf 16 pages, 8 iovs:
(00.011444) page-pipe: 		0x8048000 3
(00.011446) page-pipe: 		0xf7512000 1
(00.011447) page-pipe: 		0xf76ca000 4
(00.011449) page-pipe: 		0xf76cf000 1
(00.011450) page-pipe: 		0xf76dc000 2
(00.011452) page-pipe: 		0xf76e1000 1
(00.011454) page-pipe: 		0xf7702000 2
(00.011455) page-pipe: 		0xffdad000 2
(00.011470) page-pipe: * 0 holes:
(00.011471) PPB: 16 pages 8 segs 16 pipe 0 off
(00.011476) Sent msg to daemon 8 0 0
pie: __fetched msg: 8 0 0
(00.011479) Wait for ack 8 on daemon socket
pie: sys_vmsplice for 16 pages 8 segs 0 off
pie:  buf 16 pages, 8 iovs:
pie: 	0x8048000 0
pie: 	0x3000 0
pie: 	0xf7512000 0
pie: 	0x1000 0
pie: 	0xf76ca000 0
pie: 	0x4000 0
pie: 	0xf76cf000 0
pie: 	0x1000 0
pie: Error (pie/parasite.c:93): Can't splice pages to pipe (0/16)
pie: __sent ack msg: 8 8 -1

Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>