Comment # 5 on bug 1158111 from Chris Ward

qemu-system-ppc64 -cdrom ubuntu-19.10-server-ppc64el.iso  -m size=1024 -hda
/var/lib/libvirt/images/ubuntu19.04-ppc64le-2.qcow2 -machine
pseries-3.1,accel=tcg,usb=off,dump-guest-core=off
results in a boot from the DVD, but the console remains blank.

qemu-system-ppc64 -cdrom ubuntu-19.10-server-ppc64el.iso  -m size=1024 -hda
/var/lib/libvirt/images/ubuntu19.04-ppc64le-2.qcow2 -machine pseries-3.1
results in a boot from the DVD, and the console comes to life. I will proceed
with an install and see if it works.

So it looks as if there is some incompatibility between tumbleweed qemu's
pseries-4.1 emulation and the ubuntu DVD, which results in the 'illegal
instruction' reports.