btrfs hosts: qcow2/raw disks are created without nocow

Description

On btrfs hosts (Fedora/openSUSE default) Lima creates disk images with CoW. A qcow2 on a datacow btrfs fragments heavily over time. There is also no mention of it in the docs.

Nocow has trade-offs: on btrfs it implies nodatasum (no checksums/self-heal), and a nocow file can't be reflinked to/from a regular one - so flagging only the disk file would break the reflink copy from the download cache and limactl clone. Flagging the directories (instance dir + download cache) at creation keeps reflinks working, since nocow <-> nocow reflink is fine.

Proposal:

1. Docs/FAQ note for btrfs hosts: chattr +C on ${LIMA_HOME} and the download cache before first use, with the trade-offs.
2. Maybe: set nocow on newly created instance/cache dirs when on btrfs, as libvirt >=6.6.0 does for new storage pools (it's opt out there) - or maybe opt-in flag?

Apologies if this has already been discussed.