storage: Move disk->backingChain to the recursive disk->src.backingStore

storage: Move disk->backingChain to the recursive disk->src.backingStore

Switch over to storing of the backing chain as a recursive
virStorageSource structure.

This is a string based move. Currently the first element will be present
twice in the backing chain as currently the retrieval function stores
the parent in the newly detected chain. This will be fixed later.