This policy rejects deprecated input, and thus permits "testing the
future".  Implement it for QMP command arguments: reject commands with
deprecated ones.  Example: when QEMU is run with -compat
deprecated-input=reject, then

    {"execute": "eject", "arguments": {"device": "cd"}}

fails like this

    {"error": {"class": "GenericError", "desc": "Deprecated parameter 'device' disabled by policy"}}

When the deprecated parameter is removed, the error will change to

    {"error": {"class": "GenericError", "desc": "Parameter 'device' is unexpected"}}

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210318155519.1224118-11-armbru@redhat.com>