XenAPI: Upgrade built-VM to Trusty

If you're not using an XVA then the built-VM was previously Saucy, which is
not supported by openstack any more.

Change-Id: I1040d9e43d517582e76f2e1df787986b5ffbc42c
Merge "XenAPI: Upgrade built-VM to Trusty"
Merge branch 'master' of https://github.com/openstack-dev/devstack