I think I may have figured it out. Took me the better part of two days to get this right. Hopefully it's of use to someone.
1. Convert the img file to vdi with the qemu dd command:
"c:\Program Files\qemu\qemu-img.exe" dd -f raw -O vdi bs=4096 if=Armbian_23.8.1_Uefi-x86_jammy_current_6.1.50_minimal.img of=armbian.vdi
2. In VirtualBox, go to File -> Tools -> Virtual Media Manager. Add the VDI file as a hard disk.
3. Right click on the disk, then Copy it to a new VDI. Increase the hard disk size. I used 250GB. [See note 1]
4. Create a new VirtualBox Machine. Set the OS type to Ubuntu (Jammy Jellyfish) [see note 2]. Use the hard disk file created in the previous step.
5. Boot the machine and go through the setup instructions. The screen size is weird on the first boot. Reboot, and then it should be good to go.
[1] Without a resize the image fails to boot partway through the boot process
[2] Not sure how much the OS type setting actually matters. It seemed to also work with Linux/Debian 64-bit or Linux/Other 64-bit. I think in my initial attempts this was somehow defaulted to "Other/Other" which does not work.
