fix return of get_*_log functions in nvme module - issue 42

[robsonsmartins]

Fixes issue #42.

Changed the return of functions get_error_log(), get_firmware_log() and get_smart_log() to serde_json::Value.

In addition to correcting the mismatched return type error (Map isn't a String), the user will be able to call something like this:

let log = get_firmware_log(&device_path).expect("error getting log");
println!("temp: {:.2} ºC", log.get("temperature").and_then(Value::as_f64).unwrap_or_default() - 273.15);