Support larger NBD_CMD_WRITE_ZEROES

This is my counterproposal for:
https://lists.gnu.org/archive/html/qemu-devel/2018-02/msg02107.html
where Edgar pointed out that NBD write zero requests are rather inefficient because the NBD spec was ambiguous on whether we could exceed an advertised maximum request size even when the command carries no payload, and so for maximum compatibility with unknown NBD partners, we erred on the conservative side.

The NBD spec has since been improved to mention that a client can try a larger WRITE_ZEROES if it is prepared to handle an EINVAL, and that a server should not hang up on overlarge requests that did not include a payload. But in the process it was pointed out that even better would be improving the spec to advertise both limits (the READ/WRITE payload limit, and the larger TRIM/ZERO limit), as done in this series.

With this in place, qemu as both NBD client and server can send zero requests up to BDRV_REQUEST_MAX_BYTES (nearly 2G) in length, rather than having to subdivide the request into 32M chunks.

This series is experimental until the NBD spec is actually modified to document the feature added here; but that modification will be easier now that I have a proof-of-concept implementation.

Eric Blake (4):
nbd: Prepare for additional block sizing info nbd/client: Refactor in preparation for more limits nbd/client: Support requests of additional block sizing info nbd/server: Support requests of additional block sizing info

 include/block/nbd.h |   8 +++-
 block/nbd.c         |  15 +++++-
 nbd/client.c        | 135 +++++++++++++++++++++++++++++++++++++++++++++++-----
 nbd/common.c        |   4 ++
 nbd/server.c        |  30 ++++++++++++
 nbd/trace-events    |   4 ++
 6 files changed, 181 insertions(+), 15 deletions(-)

2.14.3