All marks are collected in a list and then they are written in
the eventpoll image as a repeated field.

This images merge reduces the amount of image files criu
generates and may simplify the fix of mentioned above issue

v2: save the original order of entries
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>