Allow to halt a VM which doesn't have a persistent storage attached

If you already have a VM running, then you install `vagrant-persistent-storage` and add a configuration to your `Vagrantfile`, you are no longer able to stop the VM.

```
17:39:29] ==> aws: Attempting graceful shutdown of VM...
[17:39:50] ==> aws: ** Detaching persistent storage **
[17:40:01] /xxx/.vagrant.d/gems/gems/vagrant-persistent-storage-0.0.13/lib/vagrant-persistent-storage/providers/virtualbox/driver/base.rb:46:in `initialize': no implicit conversion of nil into String (TypeError)
	from /xxx/.vagrant.d/gems/gems/vagrant-persistent-storage-0.0.13/lib/vagrant-persistent-storage/providers/virtualbox/driver/base.rb:46:in `new'
	from /xxx/.vagrant.d/gems/gems/vagrant-persistent-storage-0.0.13/lib/vagrant-persistent-storage/providers/virtualbox/driver/base.rb:46:in `identical_files'
	from /xxx/.vagrant.d/gems/gems/vagrant-persistent-storage-0.0.13/lib/vagrant-persistent-storage/providers/virtualbox/driver/base.rb:30:in `detach_storage'
	from /xxx/.vagrant.d/gems/gems/vagrant-persistent-storage-0.0.13/lib/vagrant-persistent-storage/action/detach_storage.rb:25:in `call'
	from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.5/lib/vagrant/action/warden.rb:34:in `call'
```