image-fuzzer: Use errors parameter of subprocess.Popen()

Instead of manually encoding stderr and stdout output, use
`errors` parameter of subprocess.Popen().  This will make
process.communicate() return unicode strings instead of bytes
objects.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20191016192430.25098-11-ehabkost@redhat.com
Message-Id: <20191016192430.25098-11-ehabkost@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

tests/image-fuzzer/runner.py

@@ -79,16 +79,13 @@ def run_app(fd, q_args):
     devnull = open('/dev/null', 'r+')
     process = subprocess.Popen(q_args, stdin=devnull,
                                stdout=subprocess.PIPE,
-                               stderr=subprocess.PIPE)
+                               stderr=subprocess.PIPE,
+                               errors='replace')
     try:
         out, err = process.communicate()
         signal.alarm(0)
-        # fd is a text file, so we need to decode the process output before
-        # writing to it.
-        # We could be simply using the `errors` parameter of subprocess.Popen(),
-        # but this will be possible only after migrating to Python 3
-        fd.write(out.decode(errors='replace'))
-        fd.write(err.decode(errors='replace'))
+        fd.write(out)
+        fd.write(err)
         fd.flush()
         return process.returncode