qapi: merge common parts of NbdServerOptions and nbd-server-start data

qapi: merge common parts of NbdServerOptions and nbd-server-start data

Instead of comment
"Keep this type consistent with the nbd-server-start arguments", we
can simply merge these things.

Note that each field of new base already has "since" tag, equal in both
original copies. So "since" information is saved.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-ID: <20250219191914.440451-1-vsementsov@yandex-team.ru>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>