port to python3

fix several bugs regarding printer properties menu:
 * port xmldriverprefs
 * fix list and models comparison in ppds.py
 * fix printerproperties problem

Signed-off-by: Ivana Hutarova Varekova <varekova@redhat.com>
fix variable name typo

Signed-off-by: Ivana Hutarova Varekova <varekova@redhat.com>
port to python3