fix(daemon): ensure the /var/lib/docker dir exists

The btrfs driver attempts to stat the /var/lib/docker directory to ensure it
exists. If it doesn't exist then it bails with an unhelpful log line:

``` 2014/05/10 00:51:30 no such file or directory
```

In 0.10 the directory was created but quickly digging through the logs I can't
tell what sort of re-ordering of code caused this regression.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com>
(github: philips)