Add support for linked clones for VirtualBox.

This allow creation of linked clones using the VirtualBox 4.3 provider:
```
config.vm.provider "virtualbox" do |vb|
    vb.use_linked_clone = true
end
```
Instead of creating a fresh machine via importing the ovf file a master VM is created once (via ovf import) and the clones are created based on this master VM. The id of the master VM is stored in a file "master_id" in the directory of the according box.

There are a few bits missing like preventing concurrent creation of master VMs for the same box, as well as maintaining the lifetime of the master VM (when shall it be destroyed and who is responsible for it?).

Also the code might need a few adaptions since this is the first time I wrote some Ruby.

However, I thought this might be a good starting point for a discussion about how this feature could be added to Vagrant.