Merge remote-tracking branch 'bonzini/nbd-next' into staging

Merge remote-tracking branch 'bonzini/nbd-next' into staging

* bonzini/nbd-next:
  nbd: add nbd_export_get_blockdev
  nbd: negotiate with named exports
  nbd: register named exports
  qemu-nbd: rewrite termination conditions to use a state machine
  nbd: add notification for closing an NBDExport
  nbd: track clients into NBDExport
  nbd: add reference counting to NBDExport
  nbd: do not leak nbd_trip coroutines when a connection is torn down
  nbd: make refcount interface public
  nbd: do not close BlockDriverState in nbd_export_close
  nbd: pass NBDClient to nbd_send_negotiate
  nbd: add more constants