Add xenserver image

By adding a separate entry for xenserver, it will enforce the gate to
cache cirros-0.3.0-x86_64-disk.vhd.tgz.

Change-Id: Ibfd4618e98f079a53fc286f5e95f18a3d658e4d2
Merge commit 'refs/changes/97/61397/2' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch
Merge commit 'refs/changes/22/61422/2' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch
Merge commit 'refs/changes/36/68136/1' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch