cloudinit: add cloudinit section for current generated config.

cloudinit: add cloudinit section for current generated config.

Instead using vm pending options for pending cloudinit generated config,

write current generated cloudinit config in a new [special:cloudinit] SECTION.

Currently, some options like vm name, nic mac address can be hotplugged,
so they are not way to know if the cloud-init disk is already updated.

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>
Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>