Btrfs: use parent subvolume ID for detecting deleted status

Use the -p option to print parent ID and do not rely on 'top level' that
may go away as is not a reliable source. Deleted subvolumes should not
appear among live subvols anymore, the parent != 0 check is a workaround
for buggy 'btrfs' tool.
Merge pull request #71 from kdave/btrfs-use-parent-not-level

Btrfs: use parent subvolume ID for detecting deleted status