- A named CPU model is written without check='none' and with
fallback='forbid': libvirt checks before the start that the host runs
it, and it is not swapped for another model.
- Mode maximum is written as that mode; it was written as a model named
maximum.
- Model names up to 64 characters: the -noTSX and -IBRS models, the ones a
mixed set of hosts needs, were refused at 20.
- The mode or model is checked against the host's capabilities when the VM
is defined, from the page and the API alike; no CPU element (QEMU's
oldest default CPU) is no longer offered.
- The default CPU mode setting offers the modes the new VM dialog has;
no-model and custom, which matched none of them, become host-model.
