add validation to prevent users from being confused about format functionality

builder/vmware/iso/config.go

@@ -167,7 +167,12 @@ func NewConfig(raws ...interface{}) (*Config, []string, error) {
 		}
 	}
 
-	if c.Format == "" {
+	if c.Format != "" {
+		if c.RemoteType != "esx5" {
+			errs = packer.MultiErrorAppend(errs,
+				fmt.Errorf("format is only valid when RemoteType=esx5"))
+		}
+	} else {
 		c.Format = "ovf"
 	}