python/machine: use subprocess.DEVNULL instead of open(os.path.devnull)

One less file resource to manage, and it helps quiet some pylint >=
2.8.0 warnings about not using a with-context manager for the open call.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-id: 20210527211715.394144-3-jsnow@redhat.com
Message-id: 20210517184808.3562549-3-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

python/qemu/machine.py

@@ -223,9 +223,8 @@ def send_fd_scm(self, fd: Optional[int] = None,
             assert fd is not None
             fd_param.append(str(fd))
 
-        devnull = open(os.path.devnull, 'rb')
         proc = subprocess.Popen(
-            fd_param, stdin=devnull, stdout=subprocess.PIPE,
+            fd_param, stdin=subprocess.DEVNULL, stdout=subprocess.PIPE,
             stderr=subprocess.STDOUT, close_fds=False
         )
         output = proc.communicate()[0]
@@ -391,7 +390,6 @@ def _launch(self) -> None:
         """
         Launch the VM and establish a QMP connection
         """
-        devnull = open(os.path.devnull, 'rb')
         self._pre_launch()
         self._qemu_full_args = tuple(
             chain(self._wrapper,
@@ -401,7 +399,7 @@ def _launch(self) -> None:
         )
         LOG.debug('VM launch command: %r', ' '.join(self._qemu_full_args))
         self._popen = subprocess.Popen(self._qemu_full_args,
-                                       stdin=devnull,
+                                       stdin=subprocess.DEVNULL,
                                        stdout=self._qemu_log_file,
                                        stderr=subprocess.STDOUT,
                                        shell=False,