test-stream-device: Remove interface before next loop

We should not reuse the same interface twice as doing so will
cause dangling pointers.
Unregister it at every iteration.

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Christophe Fergeau <cfergeau@redhat.com>

server/tests/test-stream-device.c

@@ -342,6 +342,7 @@ static void test_stream_device(TestFixture *fixture, gconstpointer user_data)
         g_assert(message_sizes_curr - message_sizes == 5 || !device_enabled);
 
         check_vmc_error_message();
+        spice_server_remove_interface(&vmc_instance.base);
     }
 }