If you remove the Get-Datacenter command then you will get a report of all of your VM's. To use file C:\temp\reports\vm_ds.csv, you can replace VMspaces.csv with C:\temp\reports\vm_ds.csv:
Get-VM |
Select-Object -Property Name,ProvisionedSpaceGB,UsedSpaceGB,@{N='FreeSpaceGB';E={$_.ProvisionedSpaceGB-$_.UsedSpaceGB}} |
Export-CSV -Path C:\temp\reports\vm_ds.csv -NoTypeInformation -UseCulture