Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2019-02-04' into staging

Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2019-02-04' into staging

nbd patches for 2019-02-04

- deprecate 'qemu-nbd --partition'
- preparation for NBD reconnect, including better logging of read errors

# gpg: Signature made Tue 05 Feb 2019 03:50:56 GMT
# gpg:                using RSA key A7A16B4A2527436A
# gpg: Good signature from "Eric Blake <eblake@redhat.com>" [full]
# gpg:                 aka "Eric Blake (Free Software Programmer) <ebb9@byu.net>" [full]
# gpg:                 aka "[jpeg image of size 6874]" [full]
# Primary key fingerprint: 71C2 CC22 B1C4 6029 27D2  F3AA A7A1 6B4A 2527 436A

* remotes/ericb/tags/pull-nbd-2019-02-04:
  block/nbd-client: rename read_reply_co to connection_co
  block/nbd-client: don't check ioc
  block/nbd-client: fix nbd_reply_chunk_iter_receive
  block/nbd-client: split connection from initialization
  block/nbd: move connection code from block/nbd to block/nbd-client
  block/nbd-client: split channel errors from export errors
  nbd: generalize usage of nbd_read
  qemu-nbd: Deprecate qemu-nbd --partition

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>