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

# By Stefan Hajnoczi (4) and Kevin Wolf (3)
# Via Kevin Wolf
* kwolf/for-anthony:
  qcow2: Fix L1 write error handling in qcow2_update_snapshot_refcount
  qcow2: Return real error in qcow2_update_snapshot_refcount
  block: clean up I/O throttling wait_time code
  block: drop duplicated slice extension code
  block: keep I/O throttling slice time constant
  block: fix I/O throttling accounting blind spot
  usb-storage: Forward serial number to scsi-disk