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/smoketest.sh

@@ -56,6 +56,8 @@ require_vm_tools() {
     command -v "$tool" &> /dev/null \
       || die "$tool isn't installed. Install coredesktop-dev's packages: bash $(sibling_dir)/update.sh"
   done
+  qemu_has_virtio_vga \
+    || die "QEMU has no virtio-vga display. Install coredesktop-dev's packages: bash $(sibling_dir)/update.sh"
   [ -r "$KVM_DEVICE" ] && [ -w "$KVM_DEVICE" ] \
     || die "Can't use $KVM_DEVICE. Turn on virtualization (VT-x/AMD-V) in the firmware settings."
 }