Refactor StorageConfig so it does not have both imagePath and Uri properties

[lstocchi]

#212 introduced a new property to the StorageConfig struct (URI) and, by doing so, now we have to check if we are dealing with a disk storage or a remote disk device by checking imagePath and Uri fields. An idea that came up in #212 (comment) was to refactor the StorageConfig struct to be extended by a DiskStorageConfig and a NetworkBlockStorageConfig.

This PR refactors the code based on that suggestion.

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