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 commit 'refs/changes/83/86083/2' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch
Merge commit 'refs/changes/74/106774/2' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch
Merge commit 'refs/changes/75/106775/1' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch
Merge commit 'refs/changes/45/107345/1' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch