nixos: add NixOS support as third bringup option with libvirt integration

This commit adds NixOS as a third bringup option alongside guestfs and
SKIP_BRINGUP, providing a declarative and reproducible way to provision
test VMs using NixOS's functional package management system with full
libvirt integration.

Libvirt Integration:
- NixOS VMs are managed through libvirt system session for professional VM lifecycle
- Uses standard libvirt networking with DHCP assignment (192.168.122.x range)
- No port forwarding needed - direct SSH access to VM IP addresses
- Standard virsh commands for VM management (start, shutdown, destroy, console)
- Integrates with existing libvirt infrastructure and monitoring tools

SSH Session Management:
- SSH keys are dynamically generated based on directory location using
  format: ~/.ssh/kdevops-nixos-<directory>-<hash>
- This ensures unique keys per kdevops instance, preventing conflicts
  between multiple deployments
- SSH config entries are automatically managed during bringup
- Direct connections to DHCP-assigned IPs (no port forwarding complexity)

Path Compatibility:
- NixOS uses different system paths than traditional Linux distributions
- Python interpreter: /run/current-system/sw/bin/python3 (not /usr/bin/python3)
- Bash shell: /run/current-system/sw/bin/bash (not /bin/bash)
- Templates automatically detect and use correct paths for NixOS

VM Management:
- Professional VM lifecycle through libvirt system session
- XML-based VM configuration with proper resource allocation
- QCOW2 disk images with virtio drivers for performance
- Automatic network configuration via libvirt's default network
- Full integration with existing libvirt monitoring and management tools

Workflow Support:
The following workflows have initial NixOS support with only
package dependency resolution, actual run time tests will be needed
later:
- fstests: Filesystem testing (XFS, Btrfs, EXT4)
- blktests: Block layer testing (NVMe, SCSI, NBD)
- selftests: Linux kernel selftests
- mmtests: Memory management performance testing
- sysbench: Database performance benchmarking
- pynfs: NFS protocol testing
- ltp: Linux Test Project
- gitr: Git regression testing

Key Features:
- Declarative configuration through Nix expressions
- Reproducible builds using Nix flakes
- Automatic dependency resolution for workflows
- Directory-based isolation for multiple kdevops instances
- Full libvirt integration for professional VM management
- Automatic SSH configuration during bringup
- Standard networking without port forwarding complexity

This implementation provides a modern, functional approach to VM
provisioning that leverages both NixOS's strengths in reproducibility
and libvirt's professional VM management capabilities.

Generated-by: Claude AI
Signed-off-by: Luis Chamberlain <[email protected]>

Author: mcgrof

.gitignore

@@ -102,3 +102,6 @@ scripts/kconfig/.nconf-cfg
 Kconfig.passthrough_libvirt.generated
 
 archive/
+
+# NixOS generated files
+nixos/generated/

defconfigs/nixos

@@ -0,0 +1,27 @@
+CONFIG_NIXOS=y
+CONFIG_LIBVIRT=y
+
+# Disable mirror setup for NixOS
+CONFIG_ENABLE_LOCAL_LINUX_MIRROR=n
+CONFIG_USE_LOCAL_LINUX_MIRROR=n
+CONFIG_INSTALL_LOCAL_LINUX_MIRROR=n
+CONFIG_MIRROR_INSTALL=n
+
+CONFIG_NIXOS_USE_FLAKES=y
+CONFIG_NIXOS_CHANNEL="nixos-unstable"
+CONFIG_NIXOS_ENABLE_WORKFLOW_DEPS=y
+CONFIG_NIXOS_LIBVIRT_SESSION_INFERENCE=y
+
+CONFIG_NIXOS_VM_MEMORY_MB=4096
+CONFIG_NIXOS_VM_DISK_SIZE_GB=20
+CONFIG_NIXOS_VM_VCPUS=4
+
+CONFIG_WORKFLOWS=y
+CONFIG_WORKFLOW_LINUX_CUSTOM=y
+
+CONFIG_BOOTLINUX=y
+CONFIG_BOOTLINUX_9P=n
+
+CONFIG_KDEVOPS_TRY_REFRESH_REPOS=y
+CONFIG_KDEVOPS_TRY_UPDATE_SYSTEMS=y
+CONFIG_KDEVOPS_TRY_INSTALL_KDEV_TOOLS=y