Support shrinking the disk image

[AkihiroSuda]

virt-sparsify should be used under the hood: https://libguestfs.org/virt-sparsify.1.html

Virt-sparsify can locate and sparsify free space in most filesystems (eg. ext2/3/4, btrfs, NTFS, etc.), and also in LVM physical volumes.

Virt-sparsify can also convert between some disk formats, for example converting a raw disk image to a thin-provisioned qcow2 image.

This tool is not available on Homebrew yet.

Integrating it to Lima is likely to depend on:

• vz: remove "basedisk"; rename "diffdisk" #1706

[jandubois]

Isn't running fstrim inside the instance doing the same thing?

I know that at least inside Alpine I can delete lots of stuff in the diffdisk, run fstrim, and du diffdisk will report that the space is no longer being used.

For that reason Rancher Desktop runs fstrim on each start/shutdown of the instance.