The JSON parser happily accepts duplicate object member names.  The
last value wins.  Reproducer #1:

    $ qemu-system-x86_64 -qmp stdio
    {"QMP": {"version": {"qemu": {"micro": 93, "minor": 0, "major": 3},
    "package": "v3.1.0-rc3-7-g87a45d86ed"}, "capabilities": []}}
    {'execute':'qmp_capabilities'}
    {"return": {}}
    {'execute':'blockdev-add','arguments':{'driver':'null-co',
     'node-name':'foo','node-name':'bar'}}
    {"return": {}}
    {'execute':'query-named-block-nodes'}
    {"return": [{ [...] "node-name": "bar" [...] }]}

Reproducer #2 is iotest 229.

Fix the parser to reject duplicates, and fix iotest 229 not to use
them.
Reported-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <20181206121743.20762-1-armbru@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
[Trailing whitespace tidied up]
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>