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

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

* bonzini/nbd-next:
  nbd: obey FUA on reads
  nbd: do not include block_int.h
  nbd: do not block in nbd_wr_sync if no data at all is available
  nbd: consistently return negative errno values
  nbd: consistently check for <0 or >=0
  nbd: consistently use ssize_t
  nbd: avoid out of bounds access to recv_coroutine array