restore: new mechanism to sync tasks

Now we have only one mutex nr_in_progress, it says how many
tasks are not restored yet. A negative value signs that someone
failed.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>