The relational syntax you are using doesn't work because the Datastore info is not a managed entity, it's just a property field. The solution is to cast it to the right type of Datastore.Info, i.e. NasDatastoreInfo in your case. Here is the syntax to use in your model:
[Model(property="info[@type='NasDatastoreInfo'].nas.remoteHost")]
// The server for NFS datastore
public var nfsServer:String;
[Model(property="info[@type='NasDatastoreInfo'].nas.remotePath")]
// The folder for NFS datastore
public var nfsFolder:String;