Comment 23 for bug 1890501

Dumping some additional context in here after talking to danpb (libvirt/QEMU) about the underlying libvirt migrateToURI3 behaviour. It looks like v1.2.20 initial introduced the libvirt behaviour of copying the source persistent domain definition across to the destination in order to ensure something is persisted when VIR_MIGRATE_PARAM_DEST_XML wasn't provided but the VIR_MIGRATE_PERSIST_DEST flag was. Later v1.3.4 then introduced VIR_MIGRATE_PARAM_PERSIST_XML to overwrite the persistent domain on the destionation.

We also found that the reason Nova is rolling back to the persistent domain during a soft reboot is due to our use of virDomainShutdown [1] and virDomainCreate [2] within _soft_reboot [3]. virDomainReboot [4] wouldn't actually cause this as libvirt doesn't allow QEMU to exit. It would also drop our requirement for transient domains entirely from Nova so is definitely something we should look into as a follow up to this.

[1] https://libvirt.org/html/libvirt-libvirt-domain.html#virDomainShutdown
[2] https://libvirt.org/html/libvirt-libvirt-domain.html#virDomainLaunch
[3] https://github.com/openstack/nova/blob/9ecefeb836964c52a5a2969b15c82b11c51d32ab/nova/virt/libvirt/driver.py#L3157-L3203
[4] https://libvirt.org/html/libvirt-libvirt-domain.html#virDomainReboot