nbd: Minimal structured read for client

nbd: Minimal structured read for client

Minimal implementation: for structured error only error_report error
message.

Note that test 83 is now more verbose, because the implementation
prints more warnings about unexpected communication errors; perhaps
future patches should tone things down by using trace messages
instead of traces, but the common case of successful communication
is no noisier than before.

Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Signed-off-by: Eric Blake <[email protected]>
Message-Id: <20171027104037[email protected]>