I haven't used the OpenGeoSuiteSDK lately but I guess you could just use the GetMap-URL which will probably look like this: http://localhost/cgi-bin/qgis_mapserv.fcgi?map=/home/hast/UA/qgis-mapnik.qgs

Have you tried the following yet?:

qgis: {
        ptype: "gxp_wmssource",
        url: "http://localhost/cgi-bin/qgis_mapserv.fcgi?map=/home/hast/UA/qgis-mapnik.qgs",
        title: "Local QGIS MapServer",
        version:"1.1.1"
    },  

    
    {
        source: "qgis",
        title: "QGIS Server Layer",
        layers: "buildings",
        opacity: 1,
        group: "overlays"
     } 

I haven't used the OpenGeoSuiteSDK lately but I guess you could just use the GetMap-URL which will probably look like this: http://localhost/cgi-bin/qgis_mapserv.fcgi?map=/home/hast/UA/qgis-mapnik.qgs

Have you tried the following yet?:

qgis: {
        ptype: "gxp_wmssource",
        url: "http://localhost/cgi-bin/qgis_mapserv.fcgi?map=/home/hast/UA/qgis-mapnik.qgs",
        version:"1.1.1"
    },  

//////////

    {
        source: "qgis",
        title: "QGIS Server Layer",
        name: "buildings",
        opacity: 1
     } 

Edit1: Just gave it a try and it works for me ;)

I also added a proxy config, as my QGIS-Server deliveres the tiles on a different port:

var app = new gxp.Viewer({
    proxy: "proxy/?url=",

...........

and added the override for ExtJS

 * @require overrides/override-ext-ajax.js