fix: Prevent comparison error with String

This fixes following error on vagrant up:

```
[core-01] Importing base box 'coreos'...
[core-01] Matching MAC address for NAT networking...
[core-01] Setting the name of the VM...
[core-01] Clearing any previously set forwarded ports...
[core-01] Clearing any previously set network interfaces...
[core-01] Preparing network interfaces based on configuration...
[core-01] Destroying VM and associated drives...
[core-01] Running cleanup tasks for 'shell' provisioner...
/Applications/Vagrant/embedded/gems/gems/vagrant-1.4.3/plugins/providers/virtualbox/action/forward_ports.rb:22:in `<=': comparison of String with 1024 failed (ArgumentError)
	from /Applications/Vagrant/embedded/gems/gems/vagrant-1.4.3/plugins/providers/virtualbox/action/forward_ports.rb:22:in `block in call'
	from /Applications/Vagrant/embedded/gems/gems/vagrant-1.4.3/plugins/providers/virtualbox/action/forward_ports.rb:21:in `each'
	from /Applications/Vagrant/embedded/gems/gems/vagrant-1.4.3/plugins/providers/virtualbox/action/forward_ports.rb:21:in `call'
```