cloudinit: allow non-root users to set ciupgrade option

cloudinit: allow non-root users to set ciupgrade option

The new ciupgrade option was missing in $cloudinitoptions in
PVE::API2::Qemu, so $check_vm_modify_config_perm defaulted to
requiring root@pam for modifying the option. To fix this, add
ciupgrade to $cloudinitoptions. This also fixes an issue where
ciupgrade was missing in the output of `qm cloudinit pending`,
as it also relies on $cloudinitoptions.

This issue was originally reported in the forum [0].

Also add a comment to avoid similar issues when adding new options in
the future.

[0]: https://forum.proxmox.com/threads/131043/

Signed-off-by: Friedrich Weber <f.weber@proxmox.com>