vmware-vmx clarify VMware Fusion support

Cloning is a feature of VMware Fusion Professional.

Using the `vmware-vmx` builder with VMware Fusion (non-pro) will yield `Build 'vmware-vmx' errored: VMware error:` as per https://github.com/mitchellh/packer/issues/1053.
The underlying error is: `Error: One of the parameters was invalid`

This is due to [VMware Fusion not supporting clone](http://stackoverflow.com/a/20914988).

This PR updates the docs references from VMware Fusion to VMware Fusion Professional for `vmware-vmx`.

If you'd like to word this more strongly/obviously I could include something like:

```html
<div class="alert alert-block alert-warn">
<p><strong>VMware Fusion users.</strong> VMware Fusion Professional is required, the non-professional version does not contain the clone feature.
</div>
```