Add backingstore parameters

This is mostly a patch from #235, with cloning support removed.

Also I've set ``-B best`` by default (the patch in #235 just crashes when no backingstore specified). Let me know if you're not comfortable with this (the lxc by default uses ``-B dir``, but ``best`` is more sensible as it uses btrfs/zfs features if available, and fallbacks to ``dir`` if not).