Refactor StorageConfig to make code cleaner

[lstocchi]

Based on a comment made by @cfergeau on #212.
Open this issue to make that PR easier to review. I'm going to refactor it in a follow-up PR

Only a comment (or just me thinking out loud), I'm not asking for changes in response to this comment.
We could having both ImagePath and URI in StorageConfig, and avoid these checks with something like:

type StorageConfig struct {
	DevName   string `json:"devName"`
	ReadOnly  bool   `json:"readOnly,omitempty"`
}

type DiskStorageConfig {
    StorageConfig
    ImagePath string `json:"imagePath,omitempty"`
}

type NetworkBlockDevice {
    StorageConfig
    URI       string `json:"uri,omitempty"`
}

If it works, this makes sure we'll never get a NetworkBlockDevice with ImagePath set as it does not have this member at all.
This adds one more almost empty type though, and may be breaking unserializing json on upgrades (?).

Originally posted by @cfergeau in #212 (comment)