lxc-ubuntu: use btrfs subvolumes and snapshots

This patch adds btrfs subvolume and snapshot support to the lxc-ubuntu template:

1. If the lxc cache directory resides in a btrfs partition, create the cache rootfs as a btrfs subvolume.
2. If both the cache rootfs and the destination rootfs are btrfs subvolumes, use the btrfs snapshot feature to create the copy.

This speeds up container creation and saves disk space.