qemu_capabilities: Disable CPU models on old s390 machine types

qemu_capabilities: Disable CPU models on old s390 machine types

Starting a KVM domain on s390 with old machine type (such as
s390-ccw-virtio-2.5) and without any guest CPU model configured fails
with

    CPU models are not available: KVM doesn't support CPU models

QEMU error. This is cause by libvirt using host-model CPU as the default
CPU based on QEMU reporting "host" CPU model as being the default one
(see commit v5.9.0-402-g24d8202294: qemu: Use host-model CPU on s390 by
default). However, even though both QEMU and KVM support CPU models on
s390 and QEMU can give us the host-model CPU, we can't use it with old
machine types which only support -cpu host.

https://bugzilla.redhat.com/show_bug.cgi?id=1795651

Reported-by: Christian Ehrhardt <paelzer@gmail.com>
Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>