console: split console setup into logical parts

console: split console setup into logical parts

The lxc_console_create() function used to munge the ringbuffer setup and the
log file setup already. This made somewhat sense when we didn't have a separate
ringbuffer log file. Now it's just plain confusing. So split this into logical
helpers that future maintainers can understand:

- lxc_console_create_log_file()
- lxc_console_create_ringbuf(console);
- lxc_console_create_ringbuf_log_file(console);

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>