When a container c is on a btrfs filesystem but is directory backed, copying
the container will default to snapshot. This is because of
should_default_to_snapshot() returning true in this case because c is on a
btrfs filesystem. We should make sure that should_default_to_snapshot() only
returns true, when c itself is a btrfs subvolume.
Signed-off-by: Christian Brauner <cbrauner@suse.de>