Separate data for different backends #774
Description
It just so happens that the currently supported backends - qemu and libvirt - work with the same subsystem, and so it's possible to switch between them and keep the same instances.
If we introduce other backends, dealing with the differences between them seems overcomplicated, when we can just keep a completely separate dataset for them - switching between backends would mean working with data in a different place altogether.
The backends should have an ability to declare a data-format string which would be used to separate the data for that particular backend.
Only new backends would implement that feature.