Add nested_hypervisor key to enable/disable this feature in VM

When a VM is powered on you can enable/disable the nested hypervisor feature in the VM.
I use this to create VM to build baseboxes with packer within the guest, even 64bit boxes.

Sample configuration in Vagrantfile:

```ruby
# -*- mode: ruby -*-
# vi: set ft=ruby :

Vagrant.configure("2") do |config|

  config.vm.define :"tst" do |tst|
    tst.vm.box = "precise32"
    tst.vm.hostname = "tst"
    tst.vm.provision "shell", path: "scripts/provision.sh"
    tst.vm.provider "vcloud" do |v|
      v.nested_hypervisor = true
    end
  end

end
```