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

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

# By Paolo Bonzini (7) and others
# Via Kevin Wolf
* kwolf/for-anthony: (22 commits)
  pc: add compatibility machine types for 1.4
  blockdev: enable discard by default
  qemu-nbd: add --discard option
  blockdev: add discard suboption to -drive
  block: implement BDRV_O_UNMAP
  block: complete all IOs before .bdrv_truncate
  coroutine: trim down nesting level in perf_nesting test
  coroutine: move pooling to common code
  qemu-iotests: Test qcow2 image creation options
  qemu-iotests: Add qemu-img compare test
  qemu-img: Add compare subcommand
  qemu-img: Add "Quiet mode" option
  block: Add synchronous wrapper for bdrv_co_is_allocated_above
  block: refuse negative iops and bps values
  block: use Error in do_check_io_limits()
  qcow2: support compressed clusters in BlockFragInfo
  qemu-img: add compressed clusters to BlockFragInfo
  qemu-img: fix missing space in qemu-img check output
  qcow2: record fragmentation statistics during check
  qcow2: introduce check_refcounts_l1/l2() flags
  ...