We have been allowing javascript style comments in JSON ever
since commit 9428f2ce (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: NEric Blake <eblake@redhat.com>