virtio-vring: fix kick() to return a correct value.

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

drivers/virtio-vring.cc

@@ -295,8 +295,10 @@ namespace virtio {
             trace_virtio_kick(_q_index);
             _dev->kick(_q_index);
             _avail_added_since_kick = 0;
+            return true;
         }
-        return kicked;
+
+        return false;
     }
 
     void