Provide LD_LIBRARY_PATH to xen tests

The Xen tests require their own library paths, add an extra_env argument to
create_build_factory and then set LD_LIBRARY_PATH to the appropriate value for
each Xen test.

Signed-off-by: Charlie Shepherd <charlie@ctshepherd.com>
Merge pull request #7 from cs648/master

Provide LD_LIBRARY_PATH to xen tests