fixed writing rdp file on windows host

The last change of #3875 doesn't work on windows host calling `vagrant rdp boxname`

```
C:/HashiCorp/Vagrant/embedded/gems/gems/vagrant-1.6.3/plugins/hosts/windows/cap/rdp.rb:24:in `block (2 levels) in rdp_client': private method `puts' called for nil:NilClass (NoMethodError)
        from C:/HashiCorp/Vagrant/embedded/gems/gems/vagrant-1.6.3/plugins/hosts/windows/cap/rdp.rb:23:in `each'
```

So I fixed it for me with the following changes.

But instead of writing more and more temporary files I suggest writing the rdp file into .vagrant/machines/boxname/rdp.rdp to use the same file name for each box and stored in the local vagrant directory tree. Should I create a PR for that?