Add "DiskMode" (persistence) property to return object in Get-VNVMDiskAndRDM

[mtboren]

As requested in a comment at http://www.vnugglets.com/2013/12/get-vm-disks-and-rdms-via-powercli.html, let's add a property to the return of Get-VNVMDiskAndRDM that indicates the mode of the given disk ("DiskMode" in the Backing property of the corresponding [VMware.Vim.VirtualDisk] object).

We can call it Persistence (as it's called in a return from PowerCLI's Get-HardDisk cmdlet, or DiskMode as it is called in the properties of the VirtualDisk object itself.