archlinux template: fix lxc.rootfs for btrfs

When creating containers under btrfs, the rootfs is first extracted to
/usr/lib/lxc/rootfs directory. This makes template incorrectly set lxc.rootfs
to /usr/lib/lxc/rootfs directory in /var/lib/lxc/<CONTAINER>/config file.

By setting lxc.rootfs to $default_path/$name we avoid such problem.