hypervisor: implement base vs desktop variants with GUI packages only for desktop

- 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.

fragments/guest-agents/base.ks

@@ -0,0 +1,5 @@
+# Guest agents for virtual machines (base)
+
+%packages --exclude-weakdeps
+qemu-guest-agent
+%end

fragments/guest-agents/gui.ks

@@ -0,0 +1,5 @@
+# Guest agents for virtual machines (GUI only)
+
+%packages --exclude-weakdeps
+spice-vdagent
+%end

fragments/guest-agents/packages.ks

@@ -1,8 +0,0 @@
-# Guest agents for virtual machines
-
-%packages --exclude-weakdeps # Beginning of the packages section. Excludes weak package dependencies.
-
-qemu-guest-agent # "QEMU guest agent" The qemu-guest agent is unnecessary for a bare-metal system. However, it is included here to cover cases where this kickstart file is used to deploy a virtual machine 
-spice-vdagent # "Agent for Spice guests" The spice agent is unnecessary for a bare-metal system. However, it is included here to cover cases where this kickstart file is used to deploy a virtual machine 
-
-%end # End of the packages section