xenapi: refactor - extract image_utils

Extract GlanceImage and RawImage. With these abstractions in place, it's
much more easier to implement support for new container and image
formats.

related to blueprint xenapi-supported-image-import-export

Change-Id: Iffe363a40af046c5c2841ecd1a981c5672eca584
Merge "xenapi: refactor - extract image_utils"