Add the "boilerplate" necessary for subsequent patches to
simply drop in compat_props for pc machines 2.0 and older.

This patch contains no functional changes.
Signed-off-by: NGabriel Somlo <somlo@cmu.edu>
Acked-by: NAlexander Graf <agraf@suse.de>
Reviewed-by: NAndreas Färber <afaerber@suse.de>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>