Fix converting dbus.UInt types to string

Fix converting dbus.UInt types to string

With Python 3.8 converting these directly to string using str()
no longer works, we need to convert these to integer first.

On Python 3.8:

>>> str(dbus.Int64(1))
'dbus.Int64(1)'

On Python 3.7 (and older):
>>> str(dbus.UInt64(1))
'1'

This is probably related to removing __str__ function from method
from int (dbus.UInt is subclass of int) which happened in 3.8, see
https://docs.python.org/3.8/whatsnew/3.8.html

Signed-off-by: Vojtech Trefny <vtrefny@redhat.com>