XenAPI: Fix disk_formats verified by tempest
XenAPI: Fix disk_formats verified by tempest

lib/glance restricts the disk_formats to those supported by XenAPI, so
tempest needs a similar restriction for the disk_formats tested.

Confirmed as passing internal Citrix CI (affected tempest jobs are
currently disabled in voting XenServer CI until this change lands)

Change-Id: Iefa5b16a3fa1789ed583426ea47ebb22e6cb571e

Mahito/devstack,bigswitch/devstack,samgoon/devstack,noironetworks/devstack,group-policy/devstack,openstack-dev/devstack,mssumanth/devstack,nawawi/openstack,richliu/devstack,bigswitch/devstack,samgoon/devstack,dtroyer/devstack,mssumanth/devstack,noironetworks/devstack,richliu/devstack,bswartz/devstack,dirkmueller/devstack,bswartz/devstack,dirkmueller/devstack,dtroyer/devstack,Mahito/devstack,nawawi/openstack,openstack-dev/devstack,group-policy/devstack