Integration of libguestfs

I think a great addition to UTM would be integrating the open source libguestfs tools to manage virtual drives. There are dozens of commands: one of them, for example, is the virt-sparsify command to free up disk space on the host).

The library supposedly supports almost every commons filesystem: Linux (ext2, ext3, ext4, XFS, btrfs,...), MS Windows (VFAT, NTFS), macOS (HFS, HFS+) as well as BSD, LVM2, MBR and GPT disks.

This would allow shrinking the disk size even on OSes which lack TRIM support. Or more support for external disk formats (it supports qcow2, VirtualBox vdi format, vmdk, vhd, vhdx).

A copy of the repository can be found [here](https://github.com/libguestfs/libguestfs) on GitHub.