Install QEMU's virtio-vga display and check for it

The smoketest VMs start with -vga virtio, but Arch packages that display
apart from qemu-base, so the base image build failed with "Virtio VGA not
available". update.sh now installs qemu-hw-display-virtio-vga, and doctor
and smoketest run check QEMU has the display before anything downloads.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/doctor.sh

@@ -50,6 +50,15 @@ cmd_doctor() {
     problem "not installed: ${missing[*]}" "fix: bash $(sibling_dir)/update.sh"
   fi
 
+  if command -v qemu-system-x86_64 &> /dev/null; then
+    if qemu_has_virtio_vga; then
+      ok "QEMU has the virtio-vga display the smoketest's VMs use"
+    else
+      problem "QEMU has no virtio-vga display, so the smoketest's VMs won't start" \
+        "fix: bash $(sibling_dir)/update.sh"
+    fi
+  fi
+
   if [ -r /dev/kvm ] && [ -w /dev/kvm ]; then
     ok "KVM is available for the smoketest's virtual machines"
   else