block/nbd: drop BDRVNBDState::sioc

Currently sioc pointer is used just to pass from socket-connection to
nbd negotiation. Drop the field, and use local variables instead. With
next commit we'll update nbd/client-connection.c to behave
appropriately (return only top-most ioc, not two channels).

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210610100802.5888-26-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>