globals: Allow global properties to be optional

Making some global properties optional will let us simplify
compat code when a given property works on most (but not all)
subclasses of a given type.

Device types will be able to opt out from optional compat
properties by simply not registering those properties.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>