xenapi: Rename imageupload image

The imageupload driver concept has helped detangle some of the image managment
code in vm_utils.

It would be nice to extend that benefit to download as well, with an
additional benefit of being able to share more code between the upload and
download paths.

This is the first in a series of patches that aims to to do just that. The
first step is change the name so that it's not just 'imageupload' but
'image'es in general that the driver is managing.

DocImpact

Change-Id: Ia18b82568147a9a40b0da67d74bf20b12d08b9a9
Merge "xenapi: Rename imageupload image"