Adding methods to easily add/remove vmdk disks to/from VM

Create several new methods, specifically add_disk and remove_disk to allow disks to be easily added and removed to/from a VirtualMachine.

We're also adding a few other methods to Datastore class to find certain files and information about those files. Also methods to the VirtualMachine class to find all controllers and a specific disk.

Tested against vSphere 5.1 with VMDK disks