Currently, we try to rewrite every occurrence of "backing": null into
"backing": "" in qmp_blockdev_add(). However, that breaks using the
same "backing": null construction in json:{} file names (which do not go
through qmp_blockdev_add()). Currently, these then just behave as if
the option has not been specified.
Since there is actually only one place where we evaluate the @backing
option to find out whether not to use a backing file, we can instead
just check for null there. It doesn't matter that this changes the
runtime state of the option from "" to null, because nobody really does
anything with that runtime state anyway (except put it into qemu again,
but qemu doesn't care whether it's "" or null).
And in the future, it's much better if we get it to be null in that
runtime state sooner than later -- see patch 7.
Note that it was Markus (who's away having a good time, I hope) who
proposed qobject_to(), so I guess he won't object too much to seeing the
concept having landed in his tree once he returns.
(Although he hasn't reviewed the previous iteration of this series,
which included it already.)