util: storage: Add parser for qemu's json backing pseudo-protocol

util: storage: Add parser for qemu's json backing pseudo-protocol

Add a modular parser that will allow to parse 'json' backing definitions
that are supported by qemu. The initial implementation adds support for
the 'file' driver.

Due to the approach qemu took to implement the JSON backing strings it's
possible to specify them in two approaches.

The object approach:
    json:{ "file" : { "driver":"file",
                      "filename":"/path/to/file"
                    }
         }

And a partially flattened approach:
    json:{"file.driver":"file"
          "file.filename":"/path/to/file"
         }

Both of the above are supported by qemu and by the code added in this
commit. The current implementation de-flattens the first level ('file.')
if possible and required. Other handling may be added later but
currently only one level was possible anyways.