Merge remote-tracking branch 'kwolf/for-anthony' into staging

* kwolf/for-anthony: (39 commits)
  qemu-iotests: add 036 autoclear feature bit test
  qemu-iotests: add qcow2.py set-feature-bit command
  fdc-test: introduced qtest read_without_media
  fdc: fix implied seek while there is no media in drive
  qcow2: fix autoclear image header update
  xen: Don't peek behind the BlockDriverState abstraction
  xen: Don't change -drive if=xen device name during machine init
  block: Replace bdrv_get_format() by bdrv_get_format_name()
  qemu-img: document qed format on qemu-img man page
  qemu-iotests: COW with many AIO requests on the same cluster
  qemu-iotests: Some backing file COW tests
  qcow2: Fix avail_sectors in cluster allocation code
  qcow2: Simplify calculation for COW area at the end
  qcow2: always operate caches in writeback mode
  ide: support enable/disable write cache
  block: always open drivers in writeback mode
  block: add bdrv_set_enable_write_cache
  block: copy enable_write_cache in bdrv_append
  savevm: flush after saving vm state
  block: flush in writethrough mode after writes
  ...