Test: regression test to catch cross imports.

This test is useful to catch non legal imports in lib/vdsm.
Lib modules cannot use/depend upon regular (non python package
vdsm) modules: for example lib/vdscli.py can't depend on
vdsm/guestIF.py module. This test updates the PYTHONPATH by
removing the regular vdsm modules, later it tries
to import lib modules, if there are non legal imports
an ImportError is raised. The error message reports
the traceback showing the non legal import.

Change-Id: I46b293c34e5968d21097137f3d1076f4199a5fd7
Signed-off-by: Giuseppe Vallarelli <gvallare@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/14311
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>