- Split guest-agents/packages.ks into base.ks (qemu-guest-agent) and gui.ks (spice-vdagent)
- Move hypervisor modifier from variant_type.hypervisor to dedicated modifier section
- Added hypervisor modifier with base (headless) and desktop (GUI) options
- Create hypervisor-desktop variant with desktop: gnome for GUI hypervisors
- Update generate_recipe.py to recognize hypervisor-desktop variant name
This ensures GUI packages (virt-manager, spice-vdagent) are only included
in hypervisors with desktop environments, keeping headless hypervisors minimal.