cloudinit: create disk if it does not exist on start

cloudinit: create disk if it does not exist on start

create a fixed size cloudinit disk if it is referenced in config and
does not exist. the size of the disk created when first added to the
config is reduced to 4MiB to match the one created in
commit_cloudinit_disk.
maximum file size per snippet file (network, user, meta) is increased to 1MiB.
preparation for offline migration without the cloudinit disk (that is
always regenerated on start).

also fixes #1807, although a further patch is required to change the
vmid on restore

Signed-off-by: Mira Limbeck <m.limbeck@proxmox.com>
Tested-by: Dominik Csapak <d.csapak@proxmox.com>