v2 deduplication: add auto-dedup on restore

if option --auto-dedup is set on restore, then as soon as page is
restored it will be punched from the image.

open image in O_RDWR mode

Signed-off-by: Tikhomirov Pavel <snorcht@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>