char: make null_chr_write() the default method

All chardev must implement chr_write(), but parallel and null chardev
both use null_chr_write(). Move it to the base class, so we don't need
to export the function when splitting the chardev in respective files.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>