for my general understanding. When you create a virtual machine you normally create one disk image/HD with various partitions which you use to put your guest OS (debian in my case).
For running a server you would have maybe 1-2 more HD's for your data, backups... (at least in a physical environment).
I found it natural to add these as separate .img files (1 per virtual HD) which would be analog to a physical machine. As I am pretty new to KVM (have some experience with Virtual Box) so I wonder whether you agree on this