iotests/147: ensure temporary sockets are closed before exiting

This avoids the python resource leak detector from issuing warnings
in the iotests.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

tests/qemu-iotests/147

@@ -277,6 +277,7 @@ class BuiltinNBD(NBDBlockdevAddBase):
                      } }
         self.client_test(filename, flatten_sock_addr(address), 'nbd-export')
 
+        sockfd.close()
         self._server_down()