You are confusing the workspace **URI** with the server **URL**. 

The workspace URI is a way of distinguishing different workspaces and is mostly used in WFS responses. For QGIS (and other clients) you need to provide the URL that points to the WMS endpoint. There is one main one `http://localhost:8080/geoserver/ows` which provides access to all the workspaces and layers in your GeoServer instance. GeoServer also provides [virtual services][1] which have URLs like `http://localhost:8080/geoserver/topp/wms` where the workspace name is used in the URL which limits access to just the layers within that workspace.


  [1]: http://docs.geoserver.org/stable/en/user/configuration/virtual-services.html