perl: fix memory leak in RStruct actions

The return value struct was freed using a simple free() instead of the
own cleanup function of each struct: this meant dynamically allocated
values (such as strings) were leaked.

Use the proper cleanup functions instead.

generator/perl.ml

@@ -657,7 +657,7 @@ and generate_perl_struct_code typ cols name style n =
           pr "      PUSHs (sv_2mortal (newSVnv (r->%s)));\n"
             name
   ) cols;
-  pr "      free (r);\n"
+  pr "      guestfs_free_%s (r);\n" typ
 
 (* Generate Sys/Guestfs.pm. *)
 and generate_perl_pm () =