Enable USB controller for AppVMs

Description of changes

QEMU VMs require a USB controller for hotplugging to work. MicroVM.nix automatically enables the USB controller when any static USB devices are added or when graphics.enable is set to true, but this is not the case for all AppVMs. For example, after the internal USB camera passthrough to ChromiumVM was commented out, there is no USB controller present and USB camera hotplugging no longer works. This fix addresses that issue.

Fixes SSRCSP-5500.

Checklist for things done

[x] Summary of the proposed changes in the PR description
[x] More detailed description in the commit message(s)
[x] Commits are squashed into relevant entities - avoid a lot of minimal dev time commits in the PR
[x] Contribution guidelines followed
[ ] Ghaf documentation updated with the commit - https://tiiuae.github.io/ghaf/
[ ] PR linked to architecture documentation and requirement(s) (ticket id)
[x] Test procedure described (or includes tests). Select one or more:
- [x] Tested on Lenovo X1 x86_64
- [ ] Tested on Jetson Orin NX or AGX aarch64
- [ ] Tested on Polarfire riscv64
[ ] Author has run make-checks and it passes
[ ] All automatic Github Action checks pass - see actions
[x] Author has added reviewers and removed PR draft status
[ ] Change requires full re-installation
[x] Change can be updated with nixos-rebuild ... switch

Instructions for Testing

Connect a USB camera to Lenovo X1 and make sure it's available in Chromium.

tiiuae / ghaf