nbd: BLOCK_STATUS constants

nbd: BLOCK_STATUS constants

Expose the new constants and structs that will be used by both
server and client implementations of NBD_CMD_BLOCK_STATUS (the
command is currently experimental at
https://github.com/NetworkBlockDevice/nbd/blob/extension-blockstatus/doc/proto.md
but will hopefully be stabilized soon).

Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-Id: <1518702707[email protected]>
[eblake: split from larger patch on server implementation]
Signed-off-by: Eric Blake <[email protected]>