json: reject javascript comments

json: reject javascript comments

We have been allowing javascript style comments in JSON ever
since commit 9428f2c (v0.7.5), but qemu doesn't send them, and
they are not strict JSON.  Reject them for now; if we can later
prove that it is worthwhile, we can reinstate it at that point
(or even make it conditional, by adding a bool parameter to
the libvirt entry point).

* src/util/virjson.c (virJSONValueFromString): Don't enable
comment parsing.
* tests/jsontest.c (mymain): Test it.

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