page-xfer: dump compatible iovec

Change page_xfer_dump_pages the way it could handle compatible iovs.
Separated hole dumping in page_xfer_dump_hole and introduced
iterator function get_iov which will return native iovec,
converting compatible iovec if needed.

Fixes:
(00.009060) Fetched ack: 7 7 0
(00.009061) Transfering pages:
(00.009062) 	buf 16/16
(00.009063) 	p 0x100008048000 [32841]
(00.009101) Error (page-xfer.c:504): Only 65536 of 17592320561152 bytes have been spliced
(00.009253) page-pipe: Killing page pipe
(00.009263) ----------------------------------------

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>