pty logging only works correctly when stdout and stderr refer to a pty. If they
do not, we do not dup2() them and lxc_console_cb_con() will never write to the
corresponding log file descriptor.

When redirection on stdout and stderr is used we can safely assume that the user
is already logging to a file or /dev/null and creating an additional pty log
doesn't seem to make sense.
Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>