In the Karaf console, what do you see when you run this?
list -s | grep xstream
In a recent 2.5 snapshot (build 1587), I see this.
openhab> list -s | grep xstream
65 │ Active │ 80 │ 1.4.7.1 │ org.apache.servicemix.bundles.xstream
In an older 2.5 snapshot (build 1502), I see nothing.
If this bundle is not installed, try dropping this jar in addons.