Merge remote-tracking branch 'bonzini/threadpool' into staging

Merge remote-tracking branch 'bonzini/threadpool' into staging

* bonzini/threadpool: (39 commits)
  raw-win32: implement native asynchronous I/O
  raw-posix: move linux-aio.c to block/
  raw-win32: add emulated AIO support
  raw-posix: rename raw-posix-aio.h, hide unavailable prototypes
  raw: merge posix-aio-compat.c into block/raw-posix.c
  block: switch posix-aio-compat to threadpool
  threadpool: do not take lock in event_notifier_ready
  aio: add generic thread-pool facility
  qemu-thread: add QemuSemaphore
  linux-aio: use event notifiers
  aio: clean up now-unused functions
  main-loop: use aio_notify for qemu_notify_event
  main-loop: use GSource to poll AIO file descriptors
  aio: call aio_notify after setting I/O handlers
  aio: add aio_notify
  aio: make AioContexts GSources
  aio: add Win32 implementation
  aio: prepare for introducing GSource-based dispatch
  aio: add non-blocking variant of aio_wait
  aio: test node->deleted before calling io_flush
  ...

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