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>

tests/doctor.bats

@@ -71,3 +71,15 @@ load test_helper
   [[ "$output" == *"Usage: coredesktop-dev doctor"* ]]
   not_called flatpak-spawn
 }
+
+@test "doctor reports QEMU without the virtio-vga display" {
+  make_working_copy
+  stub qemu-system-x86_64 'echo "name \"VGA\", bus PCI"'
+  run cdd doctor
+  [ "$status" -eq 1 ]
+  [[ "$output" == *"PROBLEM  QEMU has no virtio-vga display"* ]]
+
+  stub qemu-system-x86_64 'echo "name \"virtio-vga\", bus PCI"'
+  run cdd doctor
+  [[ "$output" == *"ok       QEMU has the virtio-vga display"* ]]
+}