Adding my own findings here after struggling with this.
Several places talk about adding a VirtIO Block disk and installing drivers for it. I did that, but that doesn't install the pass-through SCSI controller drivers, which is what's required for using the VirtIO SCSI controller. Some places also talk about adding so and so on a "different controller". I found this confusing, because you can only choose one controller for the whole machine. I later realized that this was referring to SCSI vs SATA (or something else), rather than different SCSI controllers.
So, here's what worked for me:
- Set the SCSI controller to VirtIO SCSI single.
- Import the disks as SATA. (select Prepare for VirtIO-SCSI if using web interface)
- Add a temporary SCSI disk.
- Mount the VirtIO driver ISO. (add a SATA CD drive if necessary)
- Start the virtual machine.
- Open Device manager and install drivers for the SCSI Controller, or just use the guest tools installer to install everything.
- Shut the virtual machine down.
- Make a note of your boot order.
- Reattach your SATA disks as SCSI disks.
- Remove the temporary disk.
- Restore your boot order.
Everything should now work. Go ahead and start the machine. During some other trials when I got "inaccessible boot device", I was able to get it up and running by just booting into safe mode once, so that might be worth trying.