feat: Auto-initialize the host paths of shares

[lordkekz]

Hi, thanks for this amazing project!
I wanted to share a little patch here and hear your opinion if you want to add this kind of feature. I'm looking forward to your feedback!

Motivation

• Currently microvms will fail if they have a share which doesn't yet exist on the host.
• If some permissions in the share are not accessible to the microvm:kvm user they will not be accessible for the guest, leading to somewhat surprising errors.
• I don't want to manually create the shares and ensure they have proper permissions
  • e.g. with this approach I can mount runtime secrets into a VM and have them readable without adding an additional script in my config

Proposed change

• The install-microvm-... unit creates the directories for shares on the host.
• The install-microvm-... unit recursively chowns the directories for the configured runner user.
• Both changes only apply to fully declarative VMs.

Issues / TODO

• There may be some cases where mkdir is not the desired way to create the share source directories, e.g. if one wants to mount a zfs dataset or btrfs subvolume in the location.
• There's a risk of permissions of already existing shares being lost. => We could also only initialize the permissions if the directory was newly created by install-microvm

[astro]

This is a useful improvement. Can anyone think of any host setup that could be broken by this?

[astro]

Oh wait, this PR is only for declarative VMs!

To cover all use-cases, the mkdir must be added to microvm.binScripts.virtiofsd-run in nixos-modules/microvm/virtiofsd/default.nix. I think you won't need any permissions mangling there.