Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2019-09-05-v2' into staging

Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2019-09-05-v2' into staging

nbd patches for 2019-09-05

- Advertise NBD_FLAG_CAN_MULTI_CONN on readonly images
- Tolerate larger set of server error responses during handshake
- More precision on handling fallocate() failures due to alignment
- Better documentation of NBD connection URIs
- Implement new extension NBD_CMD_FLAG_FAST_ZERO to benefit qemu-img convert

# gpg: Signature made Thu 05 Sep 2019 22:08:17 BST
# gpg:                using RSA key 71C2CC22B1C4602927D2F3AAA7A16B4A2527436A
# 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-2019-09-05-v2:
  nbd: Implement server use of NBD FAST_ZERO
  nbd: Implement client use of NBD FAST_ZERO
  nbd: Prepare for NBD_CMD_FLAG_FAST_ZERO
  nbd: Improve per-export flag handling in server
  docs: Update preferred NBD device syntax
  block: workaround for unaligned byte range in fallocate()
  nbd: Tolerate more errors to structured reply request
  nbd: Use g_autofree in a few places
  nbd: Advertise multi-conn for shared read-only connections

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