Previously, we write a "success" status but tried to parse the pid. This
meant that we wouldn't notice a successful restore but failure to parse the
pid, which was a little strange.

We still don't know the child pid, so we will end up with a restored
process tree and a running container, but at least in this case the API
will return false indicating that something failed.

We could kill(-1, 9) in this case, but since liblxc runs as root sometimes
(e.g. LXD), that would be a Very Bad Thing.
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>