Attach the cloud-init seed ISO only when the VM disk is created

cloud-init reads the seed on the first boot only, so the Linux launchers now
mount it just for the run that creates the overlay disk. The seed drive moves
from the config files to the launcher arguments.

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

qemu/fedora44/config.ini

@@ -39,17 +39,6 @@
   driver = "virtio-blk-pci"
   drive = "disk0"
 
-# cloud-init seed ISO.
-[drive "seed0"]
-  if = "none"
-  format = "raw"
-  readonly = "on"
-  file = "${seed_image}"
-
-[device "seed0-dev"]
-  driver = "virtio-blk-pci"
-  drive = "seed0"
-
 # The NIC is attached to the user-mode network "net0", which the launcher
 # defines with -netdev from qemu/fedora44/forwards.conf because a config file
 # section cannot hold more than one hostfwd.