You need NetBSD-current post 2015-10-30 to use VirtIO SCSI.

Kernel configuration

Make sure you have the following lines in your kernel configuration:

    virtio* at pci? dev ? function ?        # Virtio PCI device
    vioscsi* at virtio?                     # Virtio SCSI device

QEmu setup

1. Create 2 files one for the installation, and one for the test SCSI drive:

    dd if=/dev/zero of=install.img seek=10m count=1
    dd if=/dev/zero of=test.img seek=10m count=1
   

2. Start QEmu pointing it to your installation serial console cdrom:

    qemu-system-x86_64 \
        -m 1024 \
        -nographic \
        -drive file=install.img,media=disk,snapshot=off,format=raw \
        -drive file=boot-com.iso,media=cdrom
   

   Install as usual, choosing serial boot blocks

3. Once you are done installing you can boot QEmu from your install.img you just populated in step 2, and the test.img you created on step 1 as your virtio sd0 disk as follows:

    qemu-system-x86_64 \
        -m 1024 \
        -k en-us \
        -nographic \
        -drive file=install.img,media=disk,snapshot=off,format=raw \
        -device virtio-scsi-pci,id=scsi \
        -device scsi-hd,drive=hd \
        -drive if=none,id=hd,file=test.img,format=raw \
        -net nic,macaddr=ba:be:00:fa:ce:01,model=virtio \
        -net user,hostfwd=tcp::2003-:22 \
        -boot c