builder/virtualbox: treat guest_additions_path as a template

builder/virtualbox/step_upload_guest_additions.go

 package virtualbox
 
 import (
+	"bytes"
 	"fmt"
 	"github.com/mitchellh/multistep"
 	"github.com/mitchellh/packer/packer"
 	"os"
+	"text/template"
 )
 
-// This step uploads a file containing the VirtualBox version, which
-// can be useful for various provisioning reasons.
+type guestAdditionsPathTemplate struct {
+	Version string
+}
+
+// This step uploads the guest additions ISO to the VM.
 type stepUploadGuestAdditions struct{}
 
 func (s *stepUploadGuestAdditions) Run(state map[string]interface{}) multistep.StepAction {
 	comm := state["communicator"].(packer.Communicator)
 	config := state["config"].(*config)
+	driver := state["driver"].(Driver)
 	guestAdditionsPath := state["guest_additions_path"].(string)
 	ui := state["ui"].(packer.Ui)
 
+	version, err := driver.Version()
+	if err != nil {
+		state["error"] = fmt.Errorf("Error reading version for guest additions upload: %s", err)
+		return multistep.ActionHalt
+	}
+
 	f, err := os.Open(guestAdditionsPath)
 	if err != nil {
 		state["error"] = fmt.Errorf("Error opening guest additions ISO: %s", err)
 		return multistep.ActionHalt
 	}
 
+	tplData := &guestAdditionsPathTemplate{
+		Version: version,
+	}
+
+	var processedPath bytes.Buffer
+	t := template.Must(template.New("path").Parse(config.GuestAdditionsPath))
+	t.Execute(&processedPath, tplData)
+
 	ui.Say("Upload VirtualBox guest additions ISO...")
-	if err := comm.Upload(config.GuestAdditionsPath, f); err != nil {
+	if err := comm.Upload(processedPath.String(), f); err != nil {
 		state["error"] = fmt.Errorf("Error uploading guest additions: %s", err)
 		return multistep.ActionHalt
 	}