tools/xl: Correct the help information and exit code of the dt-overlay command

Fix the name mismatch in the xl dt-overlay command, the
command name should be "dt-overlay" instead of "dt_overlay".
Add the missing "," in the cmdtable.

Fix the exit code of the dt-overlay command, use EXIT_FAILURE
instead of ERROR_FAIL.

Fixes: 61765a07e3 ("tools/xl: Add new xl command overlay for device tree overlay support")
Suggested-by: Anthony PERARD <anthony@xenproject.org>
Signed-off-by: Henry Wang <xin.wang2@amd.com>
Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

tools/xl/xl_cmdtable.c

@@ -635,7 +635,7 @@ const struct cmd_spec cmd_table[] = {
     { "dt-overlay",
       &main_dt_overlay, 0, 1,
       "Add/Remove a device tree overlay",
-      "add/remove <.dtbo>"
+      "add/remove <.dtbo>",
       "-h print this help\n"
     },
 #endif

tools/xl/xl_vmcontrol.c

@@ -1278,7 +1278,7 @@ int main_dt_overlay(int argc, char **argv)
     const int overlay_remove_op = 2;
 
     if (argc < 2) {
-        help("dt_overlay");
+        help("dt-overlay");
         return EXIT_FAILURE;
     }
 
@@ -1302,11 +1302,11 @@ int main_dt_overlay(int argc, char **argv)
             fprintf(stderr, "failed to read the overlay device tree file %s\n",
                     overlay_config_file);
             free(overlay_dtb);
-            return ERROR_FAIL;
+            return EXIT_FAILURE;
         }
     } else {
         fprintf(stderr, "overlay dtbo file not provided\n");
-        return ERROR_FAIL;
+        return EXIT_FAILURE;
     }
 
     rc = libxl_dt_overlay(ctx, overlay_dtb, overlay_dtb_size, op);