pc: Move compat boolean globals to PCMachineClass

This way the compat flags can be initialized in the machine_options()
function. This will help us to eventually eliminate the pc_compat_*()
functions.

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>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
