cli qmp: Mark --preconfig, exit-preconfig experimental

cli qmp: Mark --preconfig, exit-preconfig experimental

Committing to the current --preconfig / exit-preconfig interface
before it has seen any use is premature.  Mark both as experimental,
the former in documentation, the latter by renaming it to
x-exit-preconfig.

See the previous commit for more detailed rationale.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180705091402.26244-3-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Acked-by: Eduardo Habkost <ehabkost@redhat.com>
Acked-by: Igor Mammedov <imammedo@redhat.com>
[Straightforward conflict with commit 514337c142f resolved]