Update boot2docker download mechanism to the latest interface

- This fixes "GetLatestBoot2DockerReleaseURL is undefined"
- The download mechanism should cache in PreCreateCheck to align with
  other drivers

Signed-off-by: Robert Breker <robert.breker@citrix.com>

xenserver.go

@@ -288,6 +288,13 @@ func (d *Driver) GetState() (state.State, error) {
 }
 
 func (d *Driver) PreCreateCheck() error {
+	// Downloading boot2docker to cache should be done here to make sure
+	// that a download failure will not leave a machine half created.
+	b2dutils := mcnutils.NewB2dUtils(d.StorePath)
+	if err := b2dutils.UpdateISOCache(d.Boot2DockerURL); err != nil {
+		return err
+	}
+
 	return nil
 }
 
@@ -296,21 +303,8 @@ func (d *Driver) Create() error {
 
 	d.setMachineNameIfNotSet()
 
-	// Download boot2docker ISO from Internet
-	var isoURL string
 	b2dutils := mcnutils.NewB2dUtils(d.StorePath)
-
+	if err := b2dutils.CopyIsoToMachineDir(d.Boot2DockerURL, d.MachineName); err != nil {
-	if d.Boot2DockerURL != "" {
-		isoURL = d.Boot2DockerURL
-	} else {
-		isoURL, err = b2dutils.GetLatestBoot2DockerReleaseURL("")
-		if err != nil {
-			log.Errorf("Unable to check for the latest release: %s", err)
-			return err
-		}
-	}
-	log.Infof("Downloading %s from %s...", isoFilename, isoURL)
-	if err := b2dutils.DownloadISO(d.StorePath, isoFilename, isoURL); err != nil {
 		return err
 	}