api: create disks: avoid adding secondary cloud-init drives

api: create disks: avoid adding secondary cloud-init drives

This will break possibly existing workflows like
1. add second cloud-init
2. remove first cloud-init
to change the cloud-init storage.

On the other hand, it avoids unintended misconfiguration of having
mutliple cloud-init drives with potentially different settings.

Also in preparation for adding cloud-init-related API calls, where
not being able to assume that there's only one cloud-init drive/state
would complicate things quite a bit.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>