[Feature request] qemu-ga - Allow unexpected parameter

It whould be nice if the qemu-ga allowed received messages to contain fields which is not part of the spec. In my example I have a host which sends the following request:

{"execute":"guest-exec","arguments":{"path":"prl_nettool","capture-output":true,"execute-in-shell":false,"arg":[...]}}

Right now this request is rejected with the following error:

{"error": {"class": "GenericError", "desc": "Parameter 'execute-in-shell' is unexpected"}}

My situation is the hosting provider I use does have some customized solution which sends some extra arguments. I have manually patched my qemu-ga so it accepts the "execute-in-shell" parameter but I don't think this should be necessary.

Instead of "Error" it should just be a "warning" returned to the user of qemu-ga but the call should still be executed.