The statstest is xen specific. Instead of filling the code with
a huge number of #ifdef WITH_XEN, just make its entire compilation
conditional in the Makefile.am. Also ensure it links to the Xen
driver so that it builds when driver modules are enabled

* tests/Makefile.am: Make statstest xen conditional. Link to
  xen driver
* tests/Makefile.am: Remove all conditionals