builder/virtualbox: fix interpolation weirdness

builder/virtualbox/iso/builder.go

@@ -57,8 +57,12 @@ type Config struct {
 }
 
 func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
+	// Set some defaults
+	b.config.VMName = fmt.Sprintf("packer-%s-{{timestamp}}", b.config.PackerBuildName)
+
 	err := config.Decode(&b.config, &config.DecodeOpts{
-		Interpolate: true,
+		Interpolate:        true,
+		InterpolateContext: &b.config.ctx,
 		InterpolateFilter: &interpolate.RenderFilter{
 			Exclude: []string{
 				"boot_command",
@@ -112,10 +116,6 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 		b.config.ISOInterface = "ide"
 	}
 
-	if b.config.VMName == "" {
-		b.config.VMName = fmt.Sprintf("packer-%s-{{timestamp}}", b.config.PackerBuildName)
-	}
-
 	if b.config.HardDriveInterface != "ide" && b.config.HardDriveInterface != "sata" && b.config.HardDriveInterface != "scsi" {
 		errs = packer.MultiErrorAppend(
 			errs, errors.New("hard_drive_interface can only be ide, sata, or scsi"))