builder/virtualbox: remap versions that don't have guest additions

builder/virtualbox/step_download_guest_additions.go

@@ -16,6 +16,11 @@ import (
 	"time"
 )
 
+var additionsVersionMap = map[string]string {
+	"4.2.1": "4.2.0",
+	"4.1.23": "4.1.22",
+}
+
 // This step uploads a file containing the VirtualBox version, which
 // can be useful for various provisioning reasons.
 //
@@ -35,6 +40,11 @@ func (s *stepDownloadGuestAdditions) Run(state map[string]interface{}) multistep
 		return multistep.ActionHalt
 	}
 
+	if newVersion, ok := additionsVersionMap[version]; ok {
+		log.Printf("Rewriting guest additions version: %s to %s", version, newVersion)
+		version = newVersion
+	}
+
 	// First things first, we get the list of checksums for the files available
 	// for this version.
 	checksumsUrl := fmt.Sprintf("http://download.virtualbox.org/virtualbox/%s/SHA256SUMS", version)