Fix a vsphere upload bug so that something is uploaded

I think that this slightly modified version of @mheidenr's PR #672 doesn't break any backwards compatibility with the current version, but makes it usable by passing a file to ovftool.

When I used
```
"post-processors": [
    {
      "type": "vsphere",
      "username": "myusername",
      "password": "mypassword",
      "datacenter": "my_datacenter",
      "datastore": "my_datastore",
      "host": "vcenter.example.com",
      "vm_folder": "folder1/folder2",
      "vm_name": "tmpl-ubuntu",
      "path_to_resource_pool": "host/intraNet/IntraServerNet/Resources/Dev%2FTest-Low",
      "vm_network": "intraserver_net",
      "only": ["vmware"]
    }
  ]
```
everything seemed to work after making this one line change.