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/common.sh

@@ -62,6 +62,12 @@ in_flatpak() {
   [ -f /.flatpak-info ] || [ -n "${FLATPAK_ID:-}" ]
 }
 
+# Whether QEMU has the display the smoketest's VMs start with (-vga virtio).
+# Arch packages it apart from qemu-base, as qemu-hw-display-virtio-vga.
+qemu_has_virtio_vga() {
+  [[ "$(qemu-system-x86_64 -device help 2> /dev/null)" == *'name "virtio-vga"'* ]]
+}
+
 # This project's folder
 sibling_dir() {
   cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd