Create portCountArg in the if statement

You can't assign to something that doesn't exist

Co-Authored-By: edwardmlyte <edwardmlyte@gmail.com>

builder/virtualbox/common/driver_4_2.go

@@ -24,7 +24,7 @@ func (d *VBox42Driver) CreateSATAController(vmName string, name string, portcoun
 		return err
 	}
 
-	portCountArg = "--portcount"
+	portCountArg := "--portcount"
 	if strings.HasPrefix(version, "0.") || strings.HasPrefix(version, "1.") || strings.HasPrefix(version, "2.") ||
 		strings.HasPrefix(version, "3.") || strings.HasPrefix(version, "4.0") || strings.HasPrefix(version, "4.1") ||
 		strings.HasPrefix(version, "4.2") {