Set ima_disk_format to vhd when hypervisor is XenServer

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
diff --git a/lib/tempest b/lib/tempest
index c492182..1bb70c1 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -290,6 +290,7 @@
     fi
     if [ "$VIRT_DRIVER" = "xenserver" ]; then
         iniset $TEMPEST_CONFIG image disk_formats "ami,ari,aki,vhd,raw,iso"
+        iniset $TEMPEST_CONFIG scenario img_disk_format vhd
     fi
 
     # Image Features