pc: Set CPU model-id on compat_props for pc <= 2.4

Instead of relying on x86_cpudef_setup() calling
qemu_hw_version(), just make old machines set model-id explicitly
on compat_props for qemu64, qemu32, and athlon. This will allow
us to eliminate x86_cpudef_setup() later.

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>