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-debian13.py

@@ -111,12 +111,13 @@ else:
         "/usr/local/share/qemu/edk2-i386-vars.fd",
     )
 daemonize = False
+download_only = False
 extra_args = []
 
 
 def usage(stream=sys.stdout):
     stream.write(f"""\
-Usage: {Path(sys.argv[0]).name} [--daemon] [--state-dir DIR] [-- QEMU_ARGUMENT ...]
+Usage: {Path(sys.argv[0]).name} [--daemon] [--download-only] [--state-dir DIR] [-- QEMU_ARGUMENT ...]
 
 Start the Debian 13 virtual machine. A writable overlay is created in the state
 directory (default: .vm/debian13/), so the downloaded base image is never
@@ -127,6 +128,7 @@ QEMU arguments override them, e.g. `-- -m 4G -smp 4`.
 
 Options:
   --daemon         Run in the background while keeping the graphical window
+  --download-only  Download the base image if it is missing, then exit
   --state-dir DIR  Directory for the VM disk, seed ISO and runtime files
                    (default: .vm/debian13; overrides VM_STATE_DIR)
   -h, --help       Show this help
@@ -151,6 +153,8 @@ while args:
     arg = args.pop(0)
     if arg == "--daemon":
         daemonize = True
+    elif arg == "--download-only":
+        download_only = True
     elif arg == "--state-dir":
         if not args:
             die(f"{arg} requires a directory argument.", 2)
@@ -175,9 +179,10 @@ pid_file = state_dir / "qemu.pid"
 log_file = state_dir / "console.log"
 qemu_config = state_dir / "qemu.ini"
 
-for command_name in (qemu_binary, "qemu-img"):
-    if shutil.which(command_name) is None:
-        die(f"{command_name} is not installed.")
+if not download_only:
+    for command_name in (qemu_binary, "qemu-img"):
+        if shutil.which(command_name) is None:
+            die(f"{command_name} is not installed.")
 
 if not base_image.is_file():
     if uses_shared_base_image:
@@ -186,6 +191,9 @@ if not base_image.is_file():
         die(f"Debian base image was not found: {base_image}")
 if not base_image.is_file():
     die(f"downloader did not create: {base_image}")
+if download_only:
+    print(f"Base image is ready: {base_image}")
+    sys.exit(0)
 for firmware_file in (uefi_code, uefi_vars_template):
     if not firmware_file.is_file():
         die(f"required UEFI firmware was not found: {firmware_file}")