Implement BtrfsUtils::get_subvolume using libbtrfsutil

Implement BtrfsUtils::get_subvolume using libbtrfsutil

The helper from btrfs_subvolid_resolve from libbtrfs can be
reimplemented by means of libbtrfsutil. btrfs_util_subvolume_path_fd is
practically equivalent to btrfs_subvolid_resolve i.e. requires the
privileged ioctl. There's another way available to all users but is slow
as it needs to iterate overall subvolumes. This is implemented as
fallback.