[FEAT] Subvolumes for virtual machines/containers directories in /var/lib

Hi,

It'd be nice if creation of subvolumes for these special directories could be supported.

• /var/lib/qemu/
• /var/lib/libvirt/
• /var/lib/xen/
• /var/lib/docker/
• /var/lib/containerd/
• /var/lib/containers (Podman & CRI-O)
• /var/lib/incus/
• /var/lib/vz/
• /var/lib/kubelet/

If I'm not mistaken, I could add this to mount.d/btrfs, but I wasn't able to find where and how to modify partman's scheme or recipe.