pc: Move compat_props setting inside *_machine_options() functions

This will simplify the DEFINE_PC_MACHINE macro, and will help us to
implement reuse of PC_COMPAT_* macros through class_init function reuse,
in the future.

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