qmp.py: Avoid overriding a builtin object

The "id" is a builtin method to get object's identity and should not be
overridden. This might bring some issues in case someone was directly
calling "cmd(..., id=id)" but I haven't found such usage on brief search
for "cmd\(.*id=".

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170818142613.32394-10-ldoktor@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>