kvm: add support for persistent hugepages

[gedia]

This is a reworked version of #1327, where I tried to address some of @apoikos 's concerns and cater to a few more use cases that are important to my organization.

Like the rest of my PRs, this is messy, ugly, hacky, and probably doesn't merit a review yet, but since it "mostly works", I thought I'd give it a go for some feedback.

Compared to my previous PR, there is now an option to start the VM with regular memory-backed RAM if not enough hugepages are available on the host. There's also a flag to make this strict, and another flag to perform NUMA optimizations, which is the most important feature of this patch to our use case.

I'll try to provide more information on this patch in the next couple of weeks here.

[gedia]

If you want to test this patch (and #1605 #1573 #1572 as well), there's build instructions for bullseye here: https://github.com/modulus-sa/ganeti/wiki