XenAPI: Always add latest Cirros euc image

Some tests, such as test_minimum_basic_scenario, assume qcow and if
a qcow image does not exit, falls back to the euc image.
This change ensures these tests work in the stock XenServer environment.

Change-Id: I718f178f7e5e0fa4eebd17a49caf5254124092e3
Merge "XenAPI: Always add latest Cirros euc image"