xenapi: Moving Glance fetch code into image/glance:download_vhd

Part of effort to stream-line the fetch image path in vm_utils.

Change-Id: I888b8edf211a7ae8a1a8bb0dad4957090cf96e55
Merge "xenapi: Moving Glance fetch code into image/glance:download_vhd"
Don't cache template if the image already exists

With this change, when an image is initialized it won't try to fetch the
template if the image already exists. We can assume that when the image
already exists on disk there is no need for getting the template because
it is only required when creating the image for the first time.

This also prevents an issue when migrating an instance whose base image
had been deleted or made private for which the migration would fail
while fetching the template image.

Fixes bug 1206887

Change-Id: Ida95cebf8bad4ab931326d226537cc111820c998