Merge remote-tracking branch 'stefanha/block' into staging

Merge remote-tracking branch 'stefanha/block' into staging

* stefanha/block:
  block: Fix how mirror_run() frees its buffer
  win32-aio: Fix how win32_aio_process_completion() frees buffer
  scsi-disk: qemu_vfree(NULL) is fine, simplify
  w32: Make qemu_vfree() accept NULL like the POSIX implementation
  sheepdog: clean up sd_aio_setup()
  sheepdog: multiplex the rw FD to flush cache
  block: clear dirty bitmap when discarding
  ide: issue discard asynchronously but serialize the pieces
  ide: fix TRIM with empty range entry
  block: make discard asynchronous
  raw: support discard on block devices
  raw-posix: remember whether discard failed
  raw-posix: support discard on more filesystems
  block: fix initialization in bdrv_io_limits_enable()
  qcow2: Fix segfault on zero-length write

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