Thanks, setting up the case on Disco (without proposed) as outlined and hitting it:

Using:
    <cpu mode='custom' match='exact' check='partial'>
      <model fallback='allow'>core2duo</model>
      <feature policy='disable' name='osxsave'/>
      <feature policy='disable' name='ospke'/>
    </cpu>

root@d:~# virsh start test-osx
error: Failed to start domain test-osx
error: internal error: process exited while connecting to monitor: 2019-05-22T06:47:37.379603Z qemu-system-x86_64: can't apply global core2duo-x86_64-cpu.osxsave=off: Property '.osxsave' not found

or if I drop osxsave

error: internal error: process exited while connecting to monitor: 2019-05-22T06:47:56.169672Z qemu-system-x86_64: can't apply global core2duo-x86_64-cpu.ospke=off: Property '.ospke' not found

Now upgrading to proposed.


root@d:~# apt install $(apt list --upgradable 2>/dev/null | awk --field-separator='/' '/^libvirt/ {print $1}' | xargs)
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Suggested packages:
  libvirt-daemon-driver-storage-gluster libvirt-daemon-driver-storage-zfs numad auditd nfs-common pm-utils radvd systemtap zfsutils
The following packages will be upgraded:
  libvirt-clients libvirt-daemon libvirt-daemon-driver-storage-rbd libvirt-daemon-system libvirt0
5 upgraded, 0 newly installed, 0 to remove and 8 not upgraded.
Need to get 4014 kB of archives.
After this operation, 0 B of additional disk space will be used.
Get:1 http://archive.ubuntu.com/ubuntu disco-proposed/main amd64 libvirt-daemon-driver-storage-rbd amd64 5.0.0-1ubuntu2.2 [68.8 kB]
Get:2 http://archive.ubuntu.com/ubuntu disco-proposed/main amd64 libvirt-daemon-system amd64 5.0.0-1ubuntu2.2 [76.3 kB]
Get:3 http://archive.ubuntu.com/ubuntu disco-proposed/main amd64 libvirt-clients amd64 5.0.0-1ubuntu2.2 [665 kB]
Get:4 http://archive.ubuntu.com/ubuntu disco-proposed/main amd64 libvirt0 amd64 5.0.0-1ubuntu2.2 [1460 kB]
Get:5 http://archive.ubuntu.com/ubuntu disco-proposed/main amd64 libvirt-daemon amd64 5.0.0-1ubuntu2.2 [1744 kB]
Fetched 4014 kB in 1s (2801 kB/s)         
Preconfiguring packages ...
(Reading database ... 46130 files and directories currently installed.)
Preparing to unpack .../libvirt-daemon-driver-storage-rbd_5.0.0-1ubuntu2.2_amd64.deb ...
Unpacking libvirt-daemon-driver-storage-rbd (5.0.0-1ubuntu2.2) over (5.0.0-1ubuntu2.1) ...
Preparing to unpack .../libvirt-daemon-system_5.0.0-1ubuntu2.2_amd64.deb ...
Unpacking libvirt-daemon-system (5.0.0-1ubuntu2.2) over (5.0.0-1ubuntu2.1) ...
Preparing to unpack .../libvirt-clients_5.0.0-1ubuntu2.2_amd64.deb ...
Unpacking libvirt-clients (5.0.0-1ubuntu2.2) over (5.0.0-1ubuntu2.1) ...
Preparing to unpack .../libvirt0_5.0.0-1ubuntu2.2_amd64.deb ...
Unpacking libvirt0:amd64 (5.0.0-1ubuntu2.2) over (5.0.0-1ubuntu2.1) ...
Preparing to unpack .../libvirt-daemon_5.0.0-1ubuntu2.2_amd64.deb ...
Unpacking libvirt-daemon (5.0.0-1ubuntu2.2) over (5.0.0-1ubuntu2.1) ...
Setting up libvirt0:amd64 (5.0.0-1ubuntu2.2) ...
Setting up libvirt-daemon (5.0.0-1ubuntu2.2) ...
Setting up libvirt-daemon-driver-storage-rbd (5.0.0-1ubuntu2.2) ...
Setting up libvirt-clients (5.0.0-1ubuntu2.2) ...
Setting up libvirt-daemon-system (5.0.0-1ubuntu2.2) ...
virtlockd.service is a disabled or a static unit, not starting it.
virtlogd.service is a disabled or a static unit, not starting it.
Setting up libvirt-daemon dnsmasq configuration.
Processing triggers for systemd (240-6ubuntu5) ...
Processing triggers for man-db (2.8.5-2) ...
Processing triggers for libc-bin (2.29-0ubuntu2) ..

No issues here.

Case now working

root@d:~# virsh start test-osx
Domain test-osx started

We see the critical parms are no more specified
  -cpu core2duo -m 512

Setting verified