qemu-io-cmds: Simplify help_oneline

help_oneline is declared and starts as:

  static void help_oneline(const char *cmd, const cmdinfo_t *ct)
  {
      if (cmd) {
          printf("%s ", cmd);
      } else {
          printf("%s ", ct->name);
          if (ct->altname) {
              printf("(or %s) ", ct->altname);
          }
      }

However, there are only two routes to help_oneline being called:

   help_f -> help_all -> help_oneline(ct->name, ct)

   help_f -> help_onecmd(argv[1], ct)

In the first case, 'cmd' and 'ct->name' are the same thing,
so it's impossible for the if (cmd) to be false and then validly
print ct->name - this is upsetting gcc
( https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96739

 )

In the second case, cmd is argv[1] and we know we've got argv[1]
so again (cmd) is non-NULL.

Simplify help_oneline by just printing cmd.
(Also strengthen argc check just to be pedantic)

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20200824102914.105619-1-dgilbert@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>