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

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

* bonzini/nbd-next:
  nbd: fixes to read-only handling
  hmp: add NBD server commands
  nbd: disallow nbd-server-add before nbd-server-start
  nbd: force read-only export for read-only devices
  nbd: fix nbd_server_stop crash when no server was running
  nbd: accept URIs
  nbd: accept relative path to Unix socket
  qemu-nbd: initialize main loop before block layer

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>