core: Add support for zero-copy migrations

[mz-pdm]

Zero-copy migrations help with migrating very large VMs (>1 TB) or busy large VMs.

Zero-copy migrations are added using a new migration policy. Since it is a new feature, there may be unanticipated problems so the policy is supposed to be used only when the already available migration policies are insufficient to migrate a VM and the user is willing to accept the risk.

Zero-copy can be used only with parallel migrations so it requires parallel migrations enabled in Engine config. If they are disabled in VM/cluster configuration, they are still used if the zero-copy policy is enabled.

Zero-copy migrations cannot be currently used with encrypted migrations. If encrypted migrations are requested, the given policy can still be used but zero-copy is disabled.

The policy uses longer downtimes since it is aimed at very large VMs and it is unlikely that short downtimes are any useful for such VMs.

It is supposed that Vdsm on the host supports zero-copy migrations, since they are used only when parallel migrations are enabled, i.e. with cluster version 4.7 by default, and it is supposed that the hosts run an up-to-date OS. If the host doesn’t support zero-copy migrations then migration API violation will be logged there and zero-copy won’t be enabled for the migration.

See also https://ovirt.org/develop/release-management/features/virt/zerocopy-migrations.html.

Bug-Url: https://bugzilla.redhat.com/2089434

[mz-pdm]

Not much tested yet, but the basic idea works.

[mz-pdm]

Many changes, it looks working now.

[mz-pdm]

Database value updated for the camelCase change.

[ljelinkova]

/ost