Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2020-02-26' into staging

Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2020-02-26' into staging

nbd patches for 2020-02-26

- ensure multiple meta contexts work
- allow leading / in export names
- fix a failure path memory leak

# gpg: Signature made Thu 27 Feb 2020 01:53:03 GMT
# 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-2020-02-26:
  block/nbd: fix memory leak in nbd_open()
  block/nbd: extract the common cleanup code
  nbd-client: Support leading / in NBD URI
  nbd: Fix regression with multiple meta contexts

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