Add --download-only to the VM launchers

The option downloads the media a launcher would fetch automatically and
exits without checking for QEMU, creating a VM disk or starting the VM:
the base image for Debian, AlmaLinux and Fedora, and the VirtIO tools
ISO for Windows 11.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YKxXLyiL7XJ3a9e8HmR2my

run-windows11.py

@@ -36,6 +36,18 @@ def is_executable(path):
     return path.is_file() and os.access(path, os.X_OK)
 
 
+def find_image_downloader():
+    if env("VM_IMAGE_HELPER"):
+        image_downloader = Path(env("VM_IMAGE_HELPER"))
+    elif is_executable(script_dir / "vm-images" / "download-images.sh"):
+        image_downloader = script_dir / "vm-images" / "download-images.sh"
+    else:
+        image_downloader = script_dir / ".." / "vm-images" / "download-images.sh"
+    if not is_executable(image_downloader):
+        die(f"shared image helper is unavailable: {image_downloader}")
+    return image_downloader
+
+
 state_dir = Path(env("WIN11_VM_STATE_DIR") or script_dir / ".vm" / "windows11")
 disk_size = env("WIN11_VM_DISK_SIZE") or "64G"
 memory = env("WIN11_VM_MEMORY") or "8G"
@@ -69,12 +81,13 @@ uefi_vars_template = pick_firmware(
 )
 daemonize = False
 install_from_iso = False
+download_only = False
 extra_args = []
 
 
 def usage(stream=sys.stdout):
     stream.write(f"""\
-Usage: {Path(sys.argv[0]).name} [--daemon] [--install-from-iso] [--state-dir DIR] [-- QEMU_ARGUMENT ...]
+Usage: {Path(sys.argv[0]).name} [--daemon] [--download-only] [--install-from-iso] [--state-dir DIR] [-- QEMU_ARGUMENT ...]
 
 If the system disk (DIR/disk.qcow2, see --state-dir) is missing, create it from
 the Windows 11 QCOW2 base image.
@@ -83,6 +96,8 @@ If it exists, start Windows normally without attaching installation media.
 
 Options:
   --daemon            Run QEMU in the background
+  --download-only     Download the VirtIO tools ISO if it is missing, then exit
+                      (the Windows ISO and base QCOW2 are supplied manually)
   --install-from-iso  Install from ISO when the system disk is missing
   --state-dir DIR     Directory for the VM disk, seed ISO and runtime files
                       (default: .vm/windows11; overrides WIN11_VM_STATE_DIR)
@@ -111,6 +126,8 @@ while args:
     arg = args.pop(0)
     if arg == "--daemon":
         daemonize = True
+    elif arg == "--download-only":
+        download_only = True
     elif arg == "--install-from-iso":
         install_from_iso = True
     elif arg == "--state-dir":
@@ -130,6 +147,22 @@ while args:
         usage(sys.stderr)
         sys.exit(2)
 
+if download_only:
+    # The VirtIO tools ISO is the only medium the launcher downloads itself.
+    if env("WIN11_VM_TOOLS_ISO"):
+        tools_iso = Path(env("WIN11_VM_TOOLS_ISO"))
+        if not tools_iso.is_file():
+            die(f"VirtIO Windows tools ISO was not found: {tools_iso}")
+    else:
+        image_downloader = find_image_downloader()
+        tools_iso = Path(capture([str(image_downloader), "--print-path", "virtio-win"]))
+        if not tools_iso.is_file():
+            run([str(image_downloader), "virtio-win"])
+        if not tools_iso.is_file():
+            die(f"downloader did not create: {tools_iso}")
+    print(f"VirtIO Windows tools ISO is ready: {tools_iso}")
+    sys.exit(0)
+
 disk_image = Path(env("WIN11_VM_DISK_IMAGE") or state_dir / "disk.qcow2")
 seed_iso = Path(env("WIN11_VM_SEED_ISO") or disk_image.parent / "seed.iso")
 uefi_vars = state_dir / "OVMF_VARS_4M.fd"
@@ -143,14 +176,7 @@ installing_from_iso = False
 if not disk_image.is_file():
     installing = True
     installing_from_iso = install_from_iso
-    if env("VM_IMAGE_HELPER"):
-        image_downloader = Path(env("VM_IMAGE_HELPER"))
-    elif is_executable(script_dir / "vm-images" / "download-images.sh"):
-        image_downloader = script_dir / "vm-images" / "download-images.sh"
-    else:
-        image_downloader = script_dir / ".." / "vm-images" / "download-images.sh"
-    if not is_executable(image_downloader):
-        die(f"shared image helper is unavailable: {image_downloader}")
+    image_downloader = find_image_downloader()
     if install_from_iso:
         install_iso = Path(env("WIN11_VM_INSTALL_ISO")
                            or capture([str(image_downloader), "--print-path", "windows11", "--iso"]))