tools: improve reporting for option errors (RHBZ#1316041)

Improve the error messages produced by C-based tools in case of issues
with the command line options:
- explicitly mention to use -a/-d (and -A/-D in virt-diff)
- when extra arguments are found, mention the correct way to pass
  options to certain command line switches (like --format)
- in virt-inspector, give a cleaner error message when neither -i nor
  any -m is specified

In all the cases, keep the extra notice to use 'TOOL --help' to get more
help with it.

rescue/rescue.c

@@ -275,14 +275,22 @@ main (int argc, char *argv[])
   assert (live == 0);
 
   /* Must be no extra arguments on the command line. */
-  if (optind != argc)
+  if (optind != argc) {
+    fprintf (stderr, _("%s: error: extra argument '%s' on command line.\n"
+             "Make sure to specify the argument for --format or --scratch "
+             "like '--format=%s'.\n"),
+             guestfs_int_program_name, argv[optind], argv[optind]);
     usage (EXIT_FAILURE);
+  }
 
   CHECK_OPTION_format_consumed;
 
   /* User must have specified some drives. */
-  if (drvs == NULL)
+  if (drvs == NULL) {
+    fprintf (stderr, _("%s: error: you must specify at least one -a or -d option.\n"),
+             guestfs_int_program_name);
     usage (EXIT_FAILURE);
+  }
 
   /* Setting "direct mode" is required for the rescue appliance. */
   if (guestfs_set_direct (g, 1) == -1)