json: fully parse input string

json: fully parse input string

I was adding a JSON test, and was shocked to find out our parser
treated the input string of "1" as invalid JSON.  It turns out
that YAJL specifically documents that it buffers input, and that
if the last input read could be a prefix to a longer token, then
you have to explicitly tell the parser that the buffer has ended
before that token will be processed.

It doesn't help that yajl 2 renamed the function from what it was
in yajl 1.

* src/util/virjson.c (virJSONValueFromString): Complete parse, in
case buffer ends in possible token prefix.
* tests/jsontest.c (mymain): Expose the problem.

Signed-off-by: Eric Blake <eblake@redhat.com>