nbd/client: refactor nbd_read_eof

Refactor nbd_read_eof to return 1 on success, 0 on eof, when no
data was read and <0 for other cases, because returned size of
read data is not actually used.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20170804151440.320927-3-vsementsov@virtuozzo.com>
[eblake: tweak function comments, rebase to test 083 enhancements]
Signed-off-by: Eric Blake <eblake@redhat.com>