util: Do not assume comma after object id

For qemu object like rng-builtin, there are no properties after id
property. We should always set comma after object id. Otherwise it will
cause trailing comma on object:
    -object rng-builtin,id=ID,
Signed-off-by: NHan Han <hhan@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>

src/util/virqemu.c

@@ -239,12 +239,14 @@ virQEMUBuildObjectCommandlineFromJSONInternal(virBufferPtr buf,
         return -1;
     }
 
-    virBufferAsprintf(buf, "%s,id=%s,", type, alias);
+    virBufferAsprintf(buf, "%s,id=%s", type, alias);
 
-    if (props &&
-        virQEMUBuildCommandLineJSON(props, buf,
+    if (props) {
+        virBufferAddLit(buf, ",");
+        if (virQEMUBuildCommandLineJSON(props, buf,
                                 virQEMUBuildCommandLineJSONArrayBitmap) < 0)
             return -1;
+    }
 
     return 0;
 }