Rename grub_disk members

Otherwise it horribly clashes with gnulib when it's
replacing open/write/read/close

Signed-off-by: Vladimir Serbinenko <phcoder@google.com>

grub-core/disk/xen/xendisk.c

@@ -249,11 +249,11 @@ grub_virtdisk_write (grub_disk_t disk, grub_disk_addr_t sector,
 static struct grub_disk_dev grub_virtdisk_dev = {
   .name = "xen",
   .id = GRUB_DISK_DEVICE_XEN,
-  .iterate = grub_virtdisk_iterate,
-  .open = grub_virtdisk_open,
-  .close = grub_virtdisk_close,
-  .read = grub_virtdisk_read,
-  .write = grub_virtdisk_write,
+  .disk_iterate = grub_virtdisk_iterate,
+  .disk_open = grub_virtdisk_open,
+  .disk_close = grub_virtdisk_close,
+  .disk_read = grub_virtdisk_read,
+  .disk_write = grub_virtdisk_write,
   .next = 0
 };