nbd patches for 2019-01-21

Add 'qemu-nbd --list' for probing a remote NBD server's advertisements.

- Eric Blake: 0/21 nbd: add qemu-nbd --list

# gpg: Signature made Mon 21 Jan 2019 22:44:27 GMT
# gpg:                using RSA key A7A16B4A2527436A
# gpg: Good signature from "Eric Blake <eblake@redhat.com>"
# gpg:                 aka "Eric Blake (Free Software Programmer) <ebb9@byu.net>"
# gpg:                 aka "[jpeg image of size 6874]"
# Primary key fingerprint: 71C2 CC22 B1C4 6029 27D2  F3AA A7A1 6B4A 2527 436A

* remotes/ericb/tags/pull-nbd-2019-01-21: (21 commits)
  iotests: Enhance 223, 233 to cover 'qemu-nbd --list'
  nbd/client: Work around 3.0 bug for listing meta contexts
  qemu-nbd: Add --list option
  nbd/client: Add meta contexts to nbd_receive_export_list()
  nbd/client: Add nbd_receive_export_list()
  nbd/client: Refactor nbd_opt_go() to support NBD_OPT_INFO
  nbd/client: Pull out oldstyle size determination
  nbd/client: Split handshake into two functions
  nbd/client: Refactor return of nbd_receive_negotiate()
  nbd/client: Split out nbd_receive_one_meta_context()
  nbd/client: Split out nbd_send_meta_query()
  nbd/client: Change signature of nbd_negotiate_simple_meta_context()
  nbd/client: Move export name into NBDExportInfo
  nbd/client: Refactor nbd_receive_list()
  qemu-nbd: Avoid strtol open-coding
  nbd/server: Favor [u]int64_t over off_t
  nbd/server: Hoist length check to qmp_nbd_server_add
  qemu-nbd: Sanity check partition bounds
  qemu-nbd: Enhance man page
  maint: Allow for EXAMPLES in texi2pod
  ...

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