guest-agent: improve help for --allow-rpcs and --block-rpcs

Currently (8.0.3+) when calling qemu-ga --help the output looks like this:

[...]
  -b, --block-rpcs  comma-separated list of RPCs to disable (no spaces,
                    use "help" to list available RPCs)
  -a, --allow-rpcs  comma-separated list of RPCs to enable (no spaces,
                    use "help" to list available RPCs)
[...]

The sentence use "help" to list available RPCs could be misunderstood in a way that the user should call qemu-ga --help to get the list of available RPCs. Instead the user has to call qemu-ga --block-rpcs=help or qemu-ga --block-rpcs=help to get the list of rpcs.

I suggest to improve the message to make this more obvious:

  -b, --block-rpcs  comma-separated list of RPCs to disable (no spaces,
                    use "--block-rpcs=help" to list available RPCs)
  -a, --allow-rpcs  comma-separated list of RPCs to enable (no spaces,
                    use "--allow-rpcs=help" to list available RPCs)

Found while testing the patch for #1505 (closed)