title: Launching with snapshot based volume fails
body: <p>I am trying to launch an instance, and I'm adding an EBS volume on top of the root volume.
  In the extra EBS volume, I'm specifying a snapshot I created.
  When I click "Launch", I immediately get the following error:</p>
  
  <pre><code>snapshotId can only be modified on EBS devices
  </code></pre>
  
  <p>Can you explain the problem please?</p>