test -d for /vagrant/.vagrant instead of /vagrant

My veewee generated osx box uses a symlink for /vagrant instead of a
directory and thus unnecessarily fails this test.
```
sudo mkdir -p /mnt/hgfs
sudo ln -s /Volumes/VMware\ Shared\ Folders/\!%vagrant
/mnt/hgfs/\!%vagrant
```
Revert "beginnings of an osx guest plugin"

This reverts commit 0ef0bbd2945ecc1674dfdaaa5f53914fc956ac58.
It is superseded by https://github.com/mitchellh/vagrant/pull/1914