xenapi: support raw tgz image download

Support the download of raw tgz images. The image's size is retrieved
by reading the first tarinfo from the stream.

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

Change-Id: Ibb7a54261b50f73100f4ab4acd59d8e7cd2901a0
Merge "xenapi: support raw tgz image download"