In the standard setting Ganeti uses a default machine which correlates to the current qemu release, so if a VM was created with Stretch it runs "pc-i440fx-2.8" and if it was created with Buster "pc-i440fx-3.1" etc.

This leads to problems during phases where the OSes of Ganeti servers are upgraded; if a VM gets created on a server running $OSVERSION+1 it would be able to get migrated to $OSVERSION, since that qemu version doesn't support it.

However, the underlying machine type didn't really change for a long time (the main idea of this type is that it provides aged "hardware" which is well supported even in old OSes), so on eqiad/codfw/codfw-test we've already set it to "pc-i440fx-2.8".

We'll need the same for esams/ulsfo/eqsin/drmrms (but we can use "pc-i440fx-3.1" as the base line here.

I've also updated the docs to add this step when creating a new cluster: https://wikitech.wikimedia.org/wiki/Ganeti#Configure_a_static_KVM_machine_type