nbd: fix whitespace in server error message

A space was missing after the option number was printed:

  Option 0x8not permitted before TLS

becomes

  Option 0x8 not permitted before TLS

This fixes

  commit 36683283
  Author: Eric Blake <eblake@redhat.com>
  Date:   Fri Oct 14 13:33:09 2016 -0500

    nbd: Send message along with server NBD_REP_ERR errors
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>
Message-Id: <20181116155325.22428-2-berrange@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
[eblake: move lone space to next line]
Signed-off-by: NEric Blake <eblake@redhat.com>

nbd/server.c

@@ -1136,7 +1136,7 @@ static int nbd_negotiate_options(NBDClient *client, uint16_t myflags,
             default:
                 ret = nbd_opt_drop(client, NBD_REP_ERR_TLS_REQD, errp,
                                    "Option 0x%" PRIx32
-                                   "not permitted before TLS", option);
+                                   " not permitted before TLS", option);
                 /* Let the client keep trying, unless they asked to
                  * quit. In this mode, we've already sent an error, so
                  * we can't ack the abort.  */