reporter-bugzilla: use base64 XMLRPC type for encoded data instead of string type

- obsolete: 'bugzilla.addAttachment' accepted base64 data in string variable
  and stored them in database decoded

- current: 'Bug.add_attachment' accepts base64 data but it must be stored in
  base64 type, otherwise server doesn't decode the data and stores them as they
  are

- related to issue #117

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Signed-off-by: Jiri Moskovcak <jmoskovc@redhat.com>