Use QEMU's standard VGA instead of virtio-vga

On Arch, -vga virtio needs qemu-hw-display-virtio-vga and also
qemu-hw-display-virtio-gpu, which that package doesn't depend on; with only
the first, QEMU segfaults at start without a message. The smoketest's VMs
are headless (or VNC), so -vga std, which qemu-base has, is enough. This
reverts 2207208's extra package and checks.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/vm.sh

@@ -76,7 +76,7 @@ vm_start() {
     -drive "file=$disk,if=virtio,format=qcow2,discard=unmap" \
     -nic "user,model=virtio-net-pci,hostfwd=tcp:127.0.0.1:$VM_PORT-:22" \
     -virtfs "local,path=$PKG_CACHE,mount_tag=pkgcache,security_model=mapped-xattr" \
-    -vga virtio \
+    -vga std \
     "${display[@]}" \
     -serial "file:$VM_DIR/serial.log" \
     -pidfile "$VM_DIR/qemu.pid" \