xenapi: Add support for different image upload drivers

This change refactors image upload code for xenapi and adds the
xenapi_image_upload_handler config option.

Preparation for bp nova-direct-image-upload
DocImpact

Change-Id: Icd4e7ef135beddca9ddcc4a880c09e5a1702a93c
Merge "xenapi: Add support for different image upload drivers"