'Folder' is the correct target type. VmFolder, HostFolder, DatastoreFolder and NetworkFolder are vRO types that represent different folder types, but on vCenter side all are of type Folder.
Object types the folder may contain can be determined by folder's childType property - see https://www.vmware.com/support/developer/vc-sdk/visdk41pubs/ApiReference/vim.Folder.html
So if you are interested only of certain folder type, you should add some code to check the value of childType property of every returned folder object and filter out the ones that do not match the type you are searching for.