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

* remotes/bonzini/nbd-next:
  nbd: Handle NBD_OPT_LIST option.
  nbd: Handle fixed new-style clients.
  nbd: Shutdown socket before closing.
  nbd: Don't validate from and len in NBD_CMD_DISC.
  nbd: Don't export a block device with no medium.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>