OK, this is actually due to the changes between u-boot-menu
4.0.1ubuntu0.1 and 4.0.2ubuntu5~20.04.1. The relevant difference is
that the menu entry now has `fdtdir
/lib/firmware/5.8.0-29-generic/device-tree/` (or similar) when it
previously didn't load a device tree at all.
I successfully worked
Hm, I actually get similar results when I revert to 5.8.0-29:
Device 0: QEMU VirtIO Block Device
Type: Hard Disk
Capacity: 71680.0 MB = 70.0 GB (146800640 x 512)
... is now current device
Scanning virtio 0:1...
Found /boot/extlinux/extlinux.conf
