func LocateUVMFolder(ctx context.Context, layerFolders []string) (string, error)
LocateUVMFolder searches a set of layer folders to determine the "uppermost" layer which has a utility VM image. The order of the layers is (for historical) reasons Read-only-layers followed by an optional read-write layer. The RO layers are in reverse order so that the upper-most RO layer is at the start, and the base OS layer is the end.