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/install.bats

@@ -14,7 +14,7 @@ config() {
   [[ "$output" == *"is the deployed copy"* ]]
   [ "$(config)" = "WORKING_COPY=$(printf '%q' "$WC")" ]
   [ "$(readlink "$HOME/.local/bin/coredesktop-dev")" = "$PROJECT/bin/coredesktop-dev" ]
-  called install_packages shellcheck bats tea qemu-base libisoburn
+  called install_packages shellcheck bats tea qemu-base qemu-hw-display-virtio-vga libisoburn
 }
 
 @test "install refuses the deployed copy however it's spelled" {