py3: introduce a compatibility helper

introduce a module to host all the compatibility
code we need to support python2 and python3 with
the same codebase.

Change-Id: I2af4ccecf72045695ea263c1fc36ef9f76a499ed
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26196
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>