The help document of qemu-nbd misses an option
Description of problem:
The "--help" option of qemu-nbd misses the option "tls-hostname".
Steps to reproduce:
1. For the option "tls-hostname", the following code appears during option parsing and modifies the tlshostname in qemu-nbd.c:760-762.

```
        case QEMU_NBD_OPT_TLSHOSTNAME:
            tlshostname = optarg;
            break;
```
Additional information:
But it does not appear in the document provided by "--help".

It may prevent users from using the relevant function.