machine: add properties to compat_props incrementaly
Switch to adding compat properties incrementaly instead of
completly overwriting compat_props per machine type.
That removes data duplication which we have due to nested
[PC|SPAPR]_COMPAT_* macros.
It also allows to set default device properties from
default foo_machine_options() hook, which will be used
in following patch for putting VMGENID device as
a function if ISA bridge on pc/q35 machines.