luxi backend: show attribute names in errors

Currently, this backend just gives:

  Error: failed to load data. Details:
  Node 'node2': Cannot convert value JSNull, error: Unable to read Double

That is because for Luxi we get an array back from the query, as
opposed to RAPI where we get a dict (and thus have the key at
hand). This patch moves the hardcoded 'convert' functions to a
genericConvert, that also takes the attribute name, with the result
that the error message is now:

  Error: failed to load data. Details:
  Node 'node2', attribute 'mtotal': Cannot convert value 'JSNull', error: Unable to read Double

which is better for debugging.

The patch also adds quotes around the value in the Utils.fromJVal
function.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Adeodato Simo <dato@google.com>