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

README.md

@@ -44,6 +44,15 @@ corresponding `*_STATE_DIR` environment variable:
 ./run-debian13.py --state-dir /srv/vms/debian13-test --daemon
 ```
 
+With `--download-only`, a launcher only downloads the media it would fetch
+automatically and then exits without creating a VM disk or starting QEMU: the
+base image for Debian, AlmaLinux and Fedora, and the VirtIO tools ISO for
+Windows 11 (whose installation ISO and base QCOW2 are supplied manually):
+
+```bash
+./run-fedora44.py --download-only
+```
+
 If `.vm/debian13/seed.iso` is missing, the launcher automatically builds it
 from `seediso/debian13/user-data`, `seediso/debian13/meta-data`, and
 `seediso/debian13/network-config` before starting the VM.