servod: remove spammy turn-down logs

As we use many empty interfaces for multi setups (v4 + X), the logs
usually contains a lot of lines that say interface xx turned down, but
that interface is fake/empty. This change keeps the logs cleaner by only
printing that info if the interface is 'real' and not a place-holder.

BUG=None
TEST=sudo servod -b grunt // v4
     ctrl-c
// observe orderly shutdown
TEST=sudo servod -b grunt // v2
     ctrl-c
// observe orderly shutdown
TEST=sudo servod -b kevin // v3
     ctrl-c
// observe orderly shutdown

Change-Id: I9fa84c8f7ea1278f0290b7dcded041e7ac146088
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/hdctools/+/2538643
Tested-by: Ruben Rodriguez Buchillon <coconutruben@chromium.org>
Reviewed-by: Brian Nemec <bnemec@chromium.org>
Commit-Queue: Ruben Rodriguez Buchillon <coconutruben@chromium.org>
diff --git a/servo/servo_server.py b/servo/servo_server.py
index 50056d8..246f152 100755
--- a/servo/servo_server.py
+++ b/servo/servo_server.py
@@ -193,8 +193,10 @@
   def close(self):
     """Servod turn down logic."""
     for i, interface in enumerate(self._interface_list):
-      self._logger.info('Turning down interface %d' % i)
-      interface.close()
+      if not isinstance(interface, _interface.empty.Empty):
+        # Only print this on real interfaces and not place holders.
+        self._logger.info('Turning down interface %d', i)
+        interface.close()
 
   def get_devices(self):
     return self._devices.values()