xl: log an error if libxl_cpupool_destroy() fails

In fact, right now, failing at destroying a cpupool is just
not reported to the user in any explicit way.

Let's log an error, as it is customary for xl in these cases.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>

tools/libxl/xl_cmdimpl.c

@@ -7582,8 +7582,10 @@ int main_cpupooldestroy(int argc, char **argv)
         return EXIT_FAILURE;
     }
 
-    if (libxl_cpupool_destroy(ctx, poolid))
+    if (libxl_cpupool_destroy(ctx, poolid)) {
+        fprintf(stderr, "Can't destroy cpupool '%s'\n", pool);
         return EXIT_FAILURE;
+    }
 
     return EXIT_SUCCESS;
 }