You're using the IDE disk controller, which is ancient and is known buggy on DragonFly BSD... So change to SATA, NVMe or virtio, and they should just work.
You were right, changed the disk controller type to AHCI and the OS works perfectly! Thanks for helping out :)
By the way, UFS is only recommended for /boot, and I think it's better to use HAMMER2 for root even for a relatively small partition in a VM. Just keep in mind to not fill it full ;)
That's the reason I chose UFS - the installer warns that small HAMMER2 filesystems need frequent cleanup, and my needs are very simple. UFS is also working fine with AHCI.
