migration: Fix race that dest preempt thread close too early
JIRA: https://issues.redhat.com/browse/RHEL-11219
Upstream: merged
This should fix an intermittent migration-test failure that could trigger with our brew build (by commit "migration: Fix race that dest preempt thread close too early"). Since at it, I also pulled in a few similar commits that will resolve some other potential races. All commits are upstreamed.
Please have a look, thanks.