Are you using VSA ? I'm also confused by your question on the DAS Storage.
Anyway, upgrading a host is in no way a danger for your VMs. Update Manager will not modify any properties of these. You only modify the host's OS : that's why you have to put your hosts in "maintenance mode".
Regarding backups, while it's more than recommended to maintain backup of your VMs, I don't see any reason to realize specific backup before patching, especially using Update Manager.
Regards,
Ludo