xenapi: Do not retry snapshot upload on 500

If Glance returns a 500 response on an initial attempt to upload
a snapshot image, it will set the image status to killed.  Any retry
attempts will fail with a 409 response.

Hence, Nova should not attempt to retry the upload in this case, which
will eliminate many pointless retries.

Closes-Bug: 1349933

Change-Id: I1a8f2416923a368b02cf8963c747ebbb24d749a1
Merge "xenapi: Do not retry snapshot upload on 500"