Comment 8 for bug 1809083

I was using a disco guest this time (as I tried B before to cover more variants)
Note: the libvirt in disco realizes we are doing tricky bullshit (withotu shared storage), but since we know it works we set --unsafe to force it trying the migration C->X and it works.

Without fix:
1. migrate Xenial -> Cosmic
$ virsh migrate --live --verbose --copy-storage-all disco-migrate qemu+ssh://10.245.71.165/system
setlocale: No such file or directory
The authenticity of host '10.245.71.165 (10.245.71.165)' can't be established.
ECDSA key fingerprint is SHA256:USOw9WaJkZbtcHayQX5AnfKYRpCCPQYvGYXxoVphop8.
Are you sure you want to continue connecting (yes/no)? yes
Migration: [100 %]

2. migrate Cosmic -> Xenial
$ virsh migrate --live --verbose --unsafe disco-migrate qemu+ssh://10.245.71.187/system
Migration: [100 %]error: operation failed: job: unexpectedly failed

In the logs I find:
source:
019-02-07 09:18:40.005+0000: initiating migration
target:
2019-02-07T09:18:44.431313Z qemu-system-ppc64: error while loading state for instance 0x0 of device 'cpu'
2019-02-07T09:18:44.432060Z qemu-system-ppc64: load of migration failed: Invalid argument

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

1. migrate Xenial -> Cosmic
$ virsh migrate --live --verbose --copy-storage-inc disco-migrate qemu+ssh://10.245.71.165/system
setlocale: No such file or directory
Migration: [100 %]

2. migrate Cosmic -> Xenial
$ virsh migrate --live --verbose --unsafe disco-migrate qemu+ssh://10.245.71.187/system
Migration: [100 %]

The guests logs are clean:
source:
9-02-07 09:25:56.185+0000: initiating migration
2019-02-07 09:26:00.705+0000: shutting down, reason=migrated
2019-02-07T09:26:00.706632Z qemu-system-ppc64: terminating on signal 15 from pid 3868 (/usr/sbin/libvirtd)
target:
(a starting qemu, nothing else)

Thereby setting verified for cosmic