Always write xml output in UTF-8 encoding, not in the encoding of the current locale

Resolves: rhbz#1120919 - abrt] ibus-table: main.py:268:main:UnicodeEncodeError: 'ascii' codec can't encode characters in position 1150-1152: ordinal not in range(128)
See: https://bugzilla.redhat.com/show_bug.cgi?id=1120919
Release v1.8.5