RSync folder synchonization on Windows isn't compatible with MSYS-port of RSync

If MSYS port of RSync tool is used on Windows for folder synchronization, it fails with error
```
rsync: change_dir "/cygdrive/c/Users/michael.kuzmin/Projects/vagrant" failed: Bad file number (9)
```

This is because MSYS expects file paths in form of `/c/Users/michael.kuzmin/Projects/vagrant`, without leading `/cygdrive` part.

The pull request adds additional check for this case.

Also it would be great if you reopen https://github.com/mitchellh/vagrant/issues/3236 and bundle rsync and openssh right in Vagrant installer.
That would significantly simplify Vagrant adoption by developers in our company.