XenAPI: Fix new useage of trueorfalse

* Ensure that Xen setup scripts will continue to function when unset
  variables are used in stackrc
* Ensure that the generic functions are sourced in all places that xenrc
  (which sources stackrc) is sourced.

Change-Id: I54eba20733c2e149621b74a1387f0bef14fca12e
Merge "XenAPI: Fix new useage of trueorfalse"