Migration cancellation replies wrong message format

The message currently returns:

{'status': {'message': 'Migration canceled',
            'code': { 'status': { 'message': 'Migration not in progress',
                      'code': 47}}},
            'progress': 1}

The content of code is wrong. This commit fixes it so the message will
look like this:

{'status': {'message': 'Migration canceled', 'code': 47}, 'progress': 1}

Change-Id: I17a2463154e9f67191a818f137a4e7158a74dbbb
Bug-Url: https://bugzilla.redhat.com/1015887
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21250
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>