qemu-ga: switch to the new error format on the wire

IMPORTANT: this BREAKS qemu-ga compatibility for the error response.

Instead of returning something like:

{ "error": { "class": "InvalidParameterValue",
             "data": {"name": "mode", "expected": "halt|powerdown|reboot" } } }

qemu-ga now returns:

 { "error": { "class": "GenericError",
              "desc": "Parameter 'mode' expects halt|powerdown|reboot" } }

Notice that this is also a bug fix, as qemu-ga wasn't returning the
human message.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
