page-xfer: Splice data into image by chunks

This is needed for the case when the target descriptor is image
cache/proxy socket.

Signed-off-by: Rodrigo Bruno <rbruno@gsd.inesc-id.pt>
Signed-off-by: Katerina Koukiou <k.koukiou@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>