Add a QUrl serverUrl method to QWebSocketServer.

It is useful in many places, esp. when constructing clients that
should connect to the server we created.

Note that the returned host address will be LocalHost instead of Any,
to make sure clients can connect to the server even on Windows.
This should hopefully resolve the CI issues.

Change-Id: I3c400ad4d785ea398cf1a1bd113c0833fda9e3bd
Reviewed-by: Kurt Pattyn <pattyn.kurt@gmail.com>