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

# By Stefan Hajnoczi (2) and others
# Via Kevin Wolf
* kwolf/for-anthony:
  virtio-blk: Do not segfault fault if failed to initialize dataplane
  qemu-iotests: add 052 BDRV_O_SNAPSHOT test
  block: fix BDRV_O_SNAPSHOT protocol detection
  qcow2: Fix segfault in qcow2_invalidate_cache
  sheepdog: show error message for halt status