When running tempest testcase test_minimum_basic_scenario will always fail
due to lack of configuration [scenario] img_disk_format=vhd in tempest.conf
This patchset is to add this configuration when XenServer is used.
Change-Id: I4b916200e6eefb62f148ec8b644fb23ffc7e00a6
Closes-Bug: #1589787
(cherry picked from commit 36afed43534e5f50e0598ff69c7ea8249c24b5c2)