qmp: Make "id" optional again even in "oob" monitors

qmp: Make "id" optional again even in "oob" monitors

Commit cf869d53172 "qmp: support out-of-band (oob) execution" made
"id" mandatory for all commands when the client accepted capability
"oob".  This is rather onerous when you play with QMP by hand, and
unnecessarily so: only out-of-band commands need an ID for reliable
matching of response to command.

Revert that part of commit cf869d53172 for now, but have documentation
advise on the need to use "id" with out-of-band commands.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180703085358.13941-8-armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>