contrib/ivshmem-server: Print "not for production" warning

The code is okay for illustrating how things work and for testing, but
its error handling make it unfit for production use.  Print a warning
to protect the innocent.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1458066895-20632-41-git-send-email-armbru@redhat.com>
