pve7to8: avoid confusing warning about required setting 'storage' for vzdump

pve7to8: avoid confusing warning about required setting 'storage' for vzdump

It's required in the schema for notes-template and protected, but when
parsing vzdump.conf, it shouldn't matter whether the storage parameter
is set or not.

The warning is ugly and users might interpret it as something that
needs to be acted upon for the upgrade:
parse error in '/etc/vzdump.conf' - 'storage': missing property - 'notes-template' requires this property\nmissing property - 'protected' requires this property

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