config: add new meta property with the VM creation time

config: add new meta property with the VM creation time

currently we only add the creation time (ctime), that was requested
as low priority wish from some users from time to time.

Note that the meta info is not available in the update API endpoints,
and at the moment the code should not change/add/delete it either in
any place.

We may want to update in on actions like clone or backup-restore in
the future, e.g., to also save the time of that event and possibly
the original source VMID, put that can be thought out later.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Acked-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Reviewed-by: Dominik Csapak <d.csapak@proxmox.com>
Tested-by: Dominik Csapak <d.csapak@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>