    # File lib/fog/vsphere/requests/compute/list_virtual_machines.rb, line 5
            def list_virtual_machines(options = {})
              # Listing all VM's can be quite slow and expensive.  Try and optimize
              # based on the available options we have.  These conditions are in
              # ascending order of  time to complete for large deployments.
              if options['instance_uuid'] then
                list_all_virtual_machines_by_instance_uuid(options)
              elsif options['folder'] then
                list_all_virtual_machines_in_folder(options)
              else
                list_all_virtual_machines
              end
            end