multiboot: Quote filename in error message

Quote filename in error message to spot possible whitespace character in
the filename and make error message more meaningful.

Signed-off-by: Adam Lackorzynski <adam@os.inf.tu-dresden.de>
Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

hw/multiboot.c

@@ -272,7 +272,7 @@ int load_multiboot(void *fw_cfg,
             mb_debug("multiboot loading module: %s\n", initrd_filename);
             mb_mod_length = get_image_size(initrd_filename);
             if (mb_mod_length < 0) {
-                fprintf(stderr, "failed to get %s image size\n", initrd_filename);
+                fprintf(stderr, "Failed to open file '%s'\n", initrd_filename);
                 exit(1);
             }