DRAFT: vm isolation

[SomeoneSerge]

Motivation:

• Hydra does not need access to GPUs. Probably doesn't even need to see the store paths coming from untrusted (Flox, nix-community) substituters (idk yet if it pulls outputs into the store, or sees the nar files).
• Builders do not need to access Hydra signing key, or postgres, or SSH host keys.

Proposal:

• Allocate VMs per builder (using vfio-pci for GPUs), and for Hydra
• Do not run nor install anything besides the vmm in the hypervisor