If you mean by a Datastore path, then you'll have to look up the disk sources in the VM, can't do it in a filter call.
You mean only find VM's that are stored like the following:
[MyDatastore] MyFolder\VM1\
[MyDatastore] MyFolder\VM2\
[MyDatastore] MyFolder2\VM3\
[MyDatastore] MyFolder\VM4\
So you'd only get VM1, VM2, VM4 if you ask for MyFolder?