Remove ability to change XML logger during a test.

Remove ability to change XML logger during a test.

It doesn't make sense to change the logger associated with a streamer in
the middle of a test run, so only allow the logger to be set when
constructing a streamer.

Change-Id: I02661de2b6071c74d10bc854cbe436581978d2d9
Reviewed-on: http://codereview.qt.nokia.com/3622
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>