Remove libvirt/qemu packages from Containerfile, use sysext

Keep only virt-manager (GUI). The libvirt daemon, qemu, and related
tools can be installed via the libvirtd-desktop sysext.

Removed: libvirt, libvirt-daemon-kvm, qemu-img, qemu-kvm,
qemu-system-*, qemu-ui-gtk, quickemu, virt-install

Saves ~500+ MB from the container image.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: shanemcd

mybox/Containerfile

@@ -83,15 +83,7 @@ RUN <<EORUN
         yq \
         zsh \
         \
-        `# Virtualization` \
-        libvirt \
-        libvirt-daemon-kvm \
-        qemu-img \
-        qemu-kvm \
-        qemu-system-$(uname -m) \
-        qemu-ui-gtk \
-        quickemu \
-        virt-install \
+        `# Virtualization (libvirt/qemu via sysext, see docs/SYSEXTS.md)` \
         virt-manager \
         \
         `# Kubernetes / containers` \