cgroup: don't use FILE* for cg property restore

Some libcs buffer writes to FILE*, which means that we error on fclose
instead of write, which makes it hard to figure out what property actually
failed writing.

Also shorten the error path a bit. Hopefully this patch will help with
debugging #118

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>