Hello!

I worked with David off-list on this, and it eventually turned out that
the problems were hardware-related.

The cluster consisted of two Librebooted x200 machines.  One that
happened to support KVM, and one that did not.  The Libreboot wiki entry
on the x200 states that there is only one known microcode revision that
works with KVM:

  https://libreboot.org/docs/hardware/x200.html#compatibility_noblobs

Interestingly, the failing machine did have the mentioned 1067a
microcode version, whereas the other (functioning) node had 10676.

The problem with 1067a manifests with QEMU starting, but not loading the
guest OS.  Live migrating a running VM to the 1067a node *did* work,
adding to the confusion.  Just "offline migration" and starting new VMs
did not.

I'm happy to report that the cluster works fine after replacing the
dysfunctional node.  :-)