core: RetrieveImageData headers - use range instead of size

On patch http://gerrit.ovirt.org/#/c/28465/ the Size header is being
replaced with the standart Range header in GET requests for downloading
image data - this patch changes the engine to use the Range header as
well.

Change-Id: Ie15ad8b48c24d3983ae352f719267356eea16572
Signed-off-by: Liron Aravot <laravot@redhat.com>