Support 'destroy' with btrfs backend

[Chromablue]

When destroying a container created using btrfs support, llxc fails with the error:

rm: cannot remove //var/lib/lxc/impact/rootfs': Directory not empty rm: skipping/var/lib/lxc/impact/rootfs', since it's on a different device
impact successfully destroyed

to fix:

btrfs subvolume delete /rootfs

should be run before / instead of the rm statements.