criu: Check for resp type to match req in criu_api

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
criu: Return responce from criu.ack_notify

This makes req/resp loop look a bit more elegant and gets rid of "no-resp"
mode in criu.send_req

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