object: express const link with link property

Let's not mix child property and link property callbacks, as this is
confusing, use LinkProperty with DIRECT flag to hold the target pointer.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200110153039.1379601-16-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>