I found this in the logs, and I have seen examples of setting the file name to [<datastoreName>] vmName_<number>, but when I'm asking for recommendedDatastores, how can I do that?
2014-07-19T22:53:44.848-05:00 [08908 error 'SoapAdapter']
--> Required property fileName is missing from data object of type VirtualDiskFlatVer2BackingInfo
-->
--> while parsing serialized DataObject of type vim.vm.device.VirtualDisk.FlatVer2BackingInfo
--> at line 1, column 798
-->
--> while parsing property "backing" of static type VirtualDeviceBackingInfo
-->
--> while parsing serialized DataObject of type vim.vm.device.VirtualDisk
--> at line 1, column 733
-->
--> while parsing property "device" of static type VirtualDevice
-->