Started on Disco with the config described in the SRU header.

2019-02-07T09:08:54.779681Z qemu-system-ppc64: warning: System page size 0x40000000 is not enabled in page_size_mask (0x11000). Performance may be slow

Installing Cosmic qemu from proposed worked fine
$ sudo apt install $(apt list --upgradable 2>&1 | awk '/^qemu/ {gsub("/.*",""); print $0}' | xargs)
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Suggested packages:
  samba vde2 openbios-ppc openhackware debootstrap
The following packages will be upgraded:
  qemu-block-extra qemu-kvm qemu-system-common qemu-system-data qemu-system-gui qemu-system-ppc qemu-utils
7 upgraded, 0 newly installed, 0 to remove and 16 not upgraded.
Need to get 10.3 MB of archives.
After this operation, 0 B of additional disk space will be used.
Get:1 http://ports.ubuntu.com/ubuntu-ports cosmic-proposed/main ppc64el qemu-utils ppc64el 1:2.12+dfsg-3ubuntu8.3 [1055 kB]
Get:2 http://ports.ubuntu.com/ubuntu-ports cosmic-proposed/main ppc64el qemu-system-common ppc64el 1:2.12+dfsg-3ubuntu8.3 [32.4 kB]
Get:3 http://ports.ubuntu.com/ubuntu-ports cosmic-proposed/main ppc64el qemu-block-extra ppc64el 1:2.12+dfsg-3ubuntu8.3 [42.4 kB]
Get:4 http://ports.ubuntu.com/ubuntu-ports cosmic-proposed/main ppc64el qemu-system-data all 1:2.12+dfsg-3ubuntu8.3 [703 kB]
Get:5 http://ports.ubuntu.com/ubuntu-ports cosmic-proposed/main ppc64el qemu-kvm ppc64el 1:2.12+dfsg-3ubuntu8.3 [12.7 kB]
Get:6 http://ports.ubuntu.com/ubuntu-ports cosmic-proposed/main ppc64el qemu-system-ppc ppc64el 1:2.12+dfsg-3ubuntu8.3 [8429 kB]
Get:7 http://ports.ubuntu.com/ubuntu-ports cosmic-proposed/main ppc64el qemu-system-gui ppc64el 1:2.12+dfsg-3ubuntu8.3 [35.0 kB]                                                                                                                                               
Fetched 10.3 MB in 7s (1440 kB/s)                                                                                                                                                                                                                                              
(Reading database ... 114642 files and directories currently installed.)
Preparing to unpack .../0-qemu-utils_1%3a2.12+dfsg-3ubuntu8.3_ppc64el.deb ...
Unpacking qemu-utils (1:2.12+dfsg-3ubuntu8.3) over (1:2.12+dfsg-3ubuntu8.2) ...
Preparing to unpack .../1-qemu-system-common_1%3a2.12+dfsg-3ubuntu8.3_ppc64el.deb ...
Unpacking qemu-system-common (1:2.12+dfsg-3ubuntu8.3) over (1:2.12+dfsg-3ubuntu8.2) ...
Preparing to unpack .../2-qemu-block-extra_1%3a2.12+dfsg-3ubuntu8.3_ppc64el.deb ...
Unpacking qemu-block-extra:ppc64el (1:2.12+dfsg-3ubuntu8.3) over (1:2.12+dfsg-3ubuntu8.2) ...
Preparing to unpack .../3-qemu-system-data_1%3a2.12+dfsg-3ubuntu8.3_all.deb ...
Unpacking qemu-system-data (1:2.12+dfsg-3ubuntu8.3) over (1:2.12+dfsg-3ubuntu8.2) ...
Preparing to unpack .../4-qemu-kvm_1%3a2.12+dfsg-3ubuntu8.3_ppc64el.deb ...
Unpacking qemu-kvm (1:2.12+dfsg-3ubuntu8.3) over (1:2.12+dfsg-3ubuntu8.2) ...
Preparing to unpack .../5-qemu-system-ppc_1%3a2.12+dfsg-3ubuntu8.3_ppc64el.deb ...
Unpacking qemu-system-ppc (1:2.12+dfsg-3ubuntu8.3) over (1:2.12+dfsg-3ubuntu8.2) ...
Preparing to unpack .../6-qemu-system-gui_1%3a2.12+dfsg-3ubuntu8.3_ppc64el.deb ...
Unpacking qemu-system-gui (1:2.12+dfsg-3ubuntu8.3) over (1:2.12+dfsg-3ubuntu8.2) ...
Setting up qemu-system-data (1:2.12+dfsg-3ubuntu8.3) ...
Setting up qemu-block-extra:ppc64el (1:2.12+dfsg-3ubuntu8.3) ...
Setting up qemu-utils (1:2.12+dfsg-3ubuntu8.3) ...
Processing triggers for man-db (2.8.4-2) ...
Setting up qemu-system-gui (1:2.12+dfsg-3ubuntu8.3) ...
Setting up qemu-system-common (1:2.12+dfsg-3ubuntu8.3) ...
Setting up qemu-system-ppc (1:2.12+dfsg-3ubuntu8.3) ...
Setting up qemu-kvm (1:2.12+dfsg-3ubuntu8.3) ...

Retest the above
- no such warning -

Full log of the guest on start now:
2019-02-07 09:23:22.608+0000: starting up libvirt version: 4.6.0, package: 2ubuntu3.2 (Christian Ehrhardt <email address hidden> Thu, 06 Dec 2018 09:16:13 +0100), qemu version: 2.12.0Debian 1:2.12+dfsg-3ubuntu8.3, kernel: 4.18.0-14-generic, hostname: lewis.maas
LC_ALL=C PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin QEMU_AUDIO_DRV=none /usr/bin/kvm -name guest=disco,debug-threads=on -S -object secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-3-disco/master-key.aes -machine pseries-bionic,accel=kvm,usb=off,dump-guest-core=off -m 1024 -mem-prealloc -mem-path /dev/hugepages/libvirt/qemu/3-disco -realtime mlock=off -smp 4,sockets=4,cores=1,threads=1 -uuid e75c07f9-e690-4a43-9a89-6a82f9f637c7 -no-user-config -nodefaults -chardev socket,id=charmonitor,fd=23,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc -no-shutdown -boot strict=on -device qemu-xhci,id=usb,bus=pci.0,addr=0x2 -drive file=/var/lib/uvtool/libvirt/images/disco.qcow,format=qcow2,if=none,id=drive-virtio-disk0 -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x3,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 -drive file=/var/lib/uvtool/libvirt/images/disco-ds.qcow,format=qcow2,if=none,id=drive-virtio-disk1 -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x4,drive=drive-virtio-disk1,id=virtio-disk1 -netdev tap,fd=25,id=hostnet0,vhost=on,vhostfd=26 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:62:0a:65,bus=pci.0,addr=0x1 -chardev pty,id=charserial0 -device spapr-vty,chardev=charserial0,id=serial0,reg=0x30000000 -device usb-kbd,id=input0,bus=usb.0,port=1 -device usb-mouse,id=input1,bus=usb.0,port=2 -vnc 127.0.0.1:0 -device VGA,id=video0,vgamem_mb=16,bus=pci.0,addr=0x6 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x5 -sandbox on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny -msg timestamp=on
2019-02-07T09:23:22.687967Z qemu-system-ppc64: -chardev pty,id=charserial0: char device redirected to /dev/pts/2 (label charserial0)


Thereby setting verified.