This enables a new capability for which support was added in Ganeti
3.0.0: postcopy-ram migration support. Enabling this flag allows Ganeti
to automatically switch to postcopy-ram when precopy-ram migration is
not completing fast, allowing us to migrate very busy machines much more
rapidly and without human intervention (eg. no need to SIGSTOP gitlab
processes when migrating gitlab-02).

https://github.com/ganeti/ganeti/blob/master/NEWS#automatic-postcopy-migration-handling-for-kvm-guests
https://wiki.qemu.org/Features/PostCopyLiveMigration