daemon: Don't need to prefix error messages with the command name.

daemon: Don't need to prefix error messages with the command name.

The RPC stubs already prefix the command name to error messages.
The daemon doesn't have to do this.  As a (small) benefit this also
makes the daemon slightly smaller.

Code in the daemon such as:

  if (argv[0] == NULL) {
    reply_with_error ("passed an empty list");
    return NULL;
  }

now results in error messages like this:

  ><fs> command ""
  libguestfs: error: command: passed an empty list

(whereas previously you would have seen ..command: command:..)