embedart: Windows fix

According to the [wiki](https://github.com/sampsyo/beets/wiki/Hacking#coding-conventions) paths need to be converted using `syspath` before being sent to the OS. I was getting [this](https://github.com/sampsyo/beets/blob/master/beetsplug/embedart.py#L154) error before when `import`ing an album, but it seems to work now.