mkosi: always mention XFS before btrfs, as it is more like ext4
mkosi: always mention XFS before btrfs, as it is more like ext4

ext4 and xfs images are pretty much put together the same way, except
for the file system used. btrfs otoh is different as instead of a GPT
partition table any subdivisions are are done as btrfs subvolumes.
Hence, let's reorder the enums a bit, and always handle ext4/xfs
together, and btrfs after that.

systemd/mkosi,systemd/mkosi