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 which have URLs like http://localhost:8080/geoserver/topp/wms where the workspace name (topp) is used in the URL which limits access to just the layers within that workspace.