compel: Add callback-based log engine

pr_out is only special left in piegen engine, the rest use
compel's pr_x output. Probably we will need to enhance it
one day to make same close to what we have in criu.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>