qemu: Fix stopping an stopped instnace

If we stop an stopped instance, it gives:

   $ capstan stop i1398662545
   Failed to stop instance: %s i1398662545
   Failed to stop instance: i1398662545
   Stopped instance: i1398662545

With this patch:

   $ capstan stop i1398662545
   Stopped instance: i1398662545

Signed-off-by: Asias He <asias@cloudius-systems.com>