Support shrinking the disk image

`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:
- https://github.com/lima-vm/lima/issues/1706