chardev: don't splatter terminal settings on exit if not previously set

The stdio chardev finalize method calls term_exit() to restore the
original terminal settings that were saved in the "oldtty" global. If
the qemu_chr_open_stdio() method exited with an error, we might not have
any original terminal settings saved in "oldtty" yet.

eg

  $ qemu-system-x86_64 -monitor stdio -daemonize
  qemu-system-x86_64: -monitor stdio: cannot use stdio with -daemonize

will cause QEMU to splatter the terminal settings with an all-zeros
"struct termios", with predictably unpleasant results. Fortunately the
existing "stdio_in_use" flag is suitable witness for whether "oldtty"
contains settings that need restoring.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20180604123043.13985-1-berrange@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

chardev/char-stdio.c

@@ -46,8 +46,10 @@ static bool stdio_echo_state;
 
 static void term_exit(void)
 {
-    tcsetattr(0, TCSANOW, &oldtty);
-    fcntl(0, F_SETFL, old_fd0_flags);
+    if (stdio_in_use) {
+        tcsetattr(0, TCSANOW, &oldtty);
+        fcntl(0, F_SETFL, old_fd0_flags);
+    }
 }
 
 static void qemu_chr_set_echo_stdio(Chardev *chr, bool echo)