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/update.bats

@@ -7,7 +7,7 @@ HOOK_MARKER="# Installed by coredesktop-dev; remove with: coredesktop-dev hooks
 @test "update installs the packages and links the command, without asking anything" {
   run bash "$PROJECT/update.sh" < /dev/null
   [ "$status" -eq 0 ]
-  called install_packages shellcheck bats tea qemu-base libisoburn
+  called install_packages shellcheck bats tea qemu-base qemu-hw-display-virtio-vga libisoburn
   [ "$(readlink "$HOME/.local/bin/coredesktop-dev")" = "$PROJECT/bin/coredesktop-dev" ]
 }