Jetty hides almost all of its implementation classes from the webapp's classloader. If you need access to them from inside your webapp, then you need to put them into your webapp's WEB-INF/lib (in your case add a dependency for jetty-util jar).
This classloading mechanism is described here: http://www.eclipse.org/jetty/documentation/current/jetty-classloading.html
BTW, the configuration for the jetty-maven-plugin from your pom.xml seems to contain some lines from web.xml (i.e. those inside the elements). I believe maven ignores configuration it doesn't understand, but perhaps things are not configured the way you think they are? Also, I don't see any setup for using your custom LoginService.