Specify the correct image parameters for XenServer
Specify the correct image parameters for XenServer

The deprecated AMI image file opts will be removed soon.
See https://review.openstack.org/#/c/338377.
So we can't use the fallback mechanism anymore. This patch is to
specify the correct image parameters for XenServer.

Change-Id: Ic287a3ed1725c42ea29022158bc9720c9a96533f

bswartz/devstack,group-policy/devstack,dtroyer/devstack,bswartz/devstack,group-policy/devstack,richliu/devstack,richliu/devstack,noironetworks/devstack,openstack-dev/devstack,dtroyer/devstack,nawawi/openstack,openstack-dev/devstack,noironetworks/devstack,nawawi/openstack