An interface can't have any instance size or callback, or itself
implement other interfaces (this is unsupported).

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180912125303.29158-1-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>