xenapi: devstack support for raw tgz image upload

Devstack will recognise the .xen-raw.tgz extensions, and upload them to
glance as raw tgz images with xen pv_mode. This change also adds "tgz" to
the recognised container formats of glance. The changes for raw tgz
support are:

    https://review.openstack.org/#/c/40908/
    https://review.openstack.org/#/c/40909/
    https://review.openstack.org/#/c/41651/

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

Change-Id: I077564587d4303291bb4f10d62bb16380b574106
Merge "xenapi: devstack support for raw tgz image upload"