Split out nbd_request_simple_option to be reused for structured reply
option.
Signed-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20171027104037.8319-10-eblake@redhat.com>