qom/object: Display more helpful message when a parent is missing

QEMU object model is scarse in documentation. Some calls are
recursive, and it might be hard to figure out even trivial issues.

We can avoid developers to waste time in a debugging session by
displaying a simple error message.

This commit is also similar to e02bdf1c ("Display more helpful
message when an object type is missing").

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20200121110349.25842-7-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>