block/nbd: merge NBDClientSession struct back to BDRVNBDState

No reason to keep it separate, it differs from others block driver
behavior and therefore confuses. Instead of generic
  'state = (State*)bs->opaque' we have to use special helper.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20190611102720.86114-4-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
1 file changed