Tshul,
I have experienced this issue exactly. The problem is that RHEL 9 and "compatible" distros, like Rocky and AlmaLinux, are stricter about CPU type.
The KVM hypervisor allows minute specification of the processor type through its "-cpu" option. In ganeti, you can set this option with the "cpu_type" backend parameter in "gnt-cluster init" or "gnt-cluster modify." For example, to set the CPU type to "IvyBridge", run:
gnt-cluster modify -H kvm:cpu_type=IvyBridge
If the CPU type across all cluster nodes is _not_ the same, pick a type representing the least common denominator, or the type of the oldest CPU in the cluster. If all the CPU's in the cluster are identical (highly recommended for for live migration), then set "cpu_type" to "host", like this:
gnt-cluster modify -H kvm:cpu_type=host
This tells KVM to use the CPU type of the ganeti host for each instance.
Don't worry about your cluster CPUs being too "old" -- I'm running new EL9 instances on 13-year-old Westmere hardware, with no problem. If you spec cpu_type appropriately, it should be fine.
So, cpu_type is the important factor here. Without it, KVM instances will default to "QEMU Virtual CPU version n.n.n" (as seen in /proc/cpuinfo) which is, apparently, unacceptable to the EL9 Linux 5 kernel.
Hope this helps.