libxl: don't continue to create the domain if the device model is not spawned

When the device model can't be spawned, rc variable is cleared in
device_model_spawn_outcome (libxl_dm.c).
In this case libxl will continue to create the domain and let it between life
and death.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>

tools/libxl/libxl_dm.c

@@ -1215,8 +1215,6 @@ static void device_model_spawn_outcome(libxl__egc *egc,
         }
     }
 
-    rc = 0;
-
  out:
     dmss->callback(egc, dmss, rc);
 }