Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2019-04-01' into staging

nbd patches for 2019-04-01

- Better behavior of qemu-img map on NBD images
- Fixes for NBD protocol alignment corner cases:
 - the server has fewer places where it sends reads or block status
   not aligned to its advertised block size
 - the client has more cases where it can work around server
   non-compliance present in qemu 3.1
 - the client now avoids non-compliant requests when interoperating
   with nbdkit or other servers not advertising block size

# gpg: Signature made Mon 01 Apr 2019 15:06:54 BST
# 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-2019-04-01:
  nbd/client: Trace server noncompliance on structured reads
  nbd/server: Advertise actual minimum block size
  block: Add bdrv_get_request_alignment()
  nbd/client: Support qemu-img convert from unaligned size
  nbd/client: Reject inaccessible tail of inconsistent server
  nbd/client: Report offsets in bdrv_block_status
  nbd/client: Lower min_block for block-status, unaligned size
  iotests: Add 241 to test NBD on unaligned images
  nbd-client: Work around server BLOCK_STATUS misalignment at EOF
  qemu-img: Gracefully shutdown when map can't finish
  nbd: Permit simple error to NBD_CMD_BLOCK_STATUS
  nbd: Don't lose server's error to NBD_CMD_BLOCK_STATUS
  nbd: Tolerate some server non-compliance in NBD_CMD_BLOCK_STATUS
  qemu-img: Report bdrv_block_status failures

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