[Fuchsia] Don't close qemu stderr

After https://crrev.com/580342 fuchsia runner scripts redirects system
log to serial port and captures if from qemu stdout. It also closes
the stdout stream after the system is booted. In some cases this
causes qemu die later. With this change the stdout pipe is redirected
to /dev/null instead of being closed.

Change-Id: I37bcc744d4ca0cb72ac1dffd3d436c3675c951eb
Reviewed-on: https://chromium-review.googlesource.com/1162864
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580728}

build/fuchsia/target.py

@@ -167,9 +167,10 @@ class Target(object):
           return True
         time.sleep(_ATTACH_RETRY_INTERVAL)
     finally:
+      # Redirect logs to /dev/null. run_package.py will use SSH+dlog to get
+      # logs from the machine to console.
       if self._system_logs_reader:
-        self._system_logs_reader.Close()
-        self._system_logs_reader = None
+        self._system_logs_reader.RedirectTo(open('/dev/null', 'w'));
 
     logging.error('Timeout limit reached.')