missing features with CPU `qemu64`
Description of problem:
The live migration complains about a missing feature when using the CPU qemu64, which is _guaranteed to work_.
Steps to reproduce:
1. start the VM with qemu64 on the CPU: Intel(R) Xeon(R) CPU E5-2620 v4 
2. live-migrate the VM to a CPU: Intel(R) Xeon(R) CPU E5-2670 0
Additional information:
The migration fails:
```
root@covid21:~# virsh migrate --verbose --live --persistent --undefinesource myvm.local qemu+ssh://covid24/system
error: operation failed: guest CPU doesn't match specification: missing features: abm
```

This should not happen on a generic CPU, which should always work. Note, that the migration succeeds when using `-cpu qemu64,abm=off …`