Merge remote-tracking branch 'remotes/kevin/tags/for-upstream' into staging

Block patches for 2.0.0-rc1

# gpg: Signature made Wed 19 Mar 2014 13:03:27 GMT using RSA key ID C88F2FD6
# gpg: Good signature from "Kevin Wolf <[email protected]>"

* remotes/kevin/tags/for-upstream:
  dataplane: fix implicit IOThread refcount
  block/nfs: report errors from libnfs
  block/nfs: bump libnfs requirement to 1.9.3
  qcow2: Fix fail path in realloc_refcount_block()
  qcow2: Correct comment for realloc_refcount_block()
  qemu-io: Extended "--cmd" description in usage text
  qemu-io-cmds: Fixed typo in example for writev.
  block: Add error handling to bdrv_invalidate_cache()

Signed-off-by: Peter Maydell <[email protected]>