ksmd should be opt-in rather than opt-out

This was originally raised in https://bugs.launchpad.net/cloud-images/+bug/2032933/comments/13 :

> > +ksmd
>$
> I'm concerned about this change. Historically, the page-merging
> code has allowed cross-VM snooping, including even recovery of
> GnuPG private keys: https://eprint.iacr.org/2013/448.pdf
>$
> Unless something has changed to mitigate the cross-domain privacy
> leaks in ksmd, it ought to be opt-in for administrators to select
> if all their VMs are in the same security domain.

There's a collection of references on Wikipedia https://en.wikipedia.org/wiki/Kernel_same-page_merging#Security_risks showing the work; there's a paper from 2021 demonstrating the issue remotely: https://graz.elsevierpure.com/en/publications/remote-memory-deduplication-attacks

I realize KSM is the difference between some workloads working and not working, so I'm not proposing that it be removed entirely. It has its place. But I also think it should be opt-in rather than opt-out.

Thanks