vmdk: Fix header structure

Commit bb45ded9 swapped gd_offset and rgd_offset. This is wrong.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/vmdk.c

@@ -57,8 +57,8 @@ typedef struct {
     int64_t desc_offset;
     int64_t desc_size;
     int32_t num_gtes_per_gte;
-    int64_t gd_offset;
     int64_t rgd_offset;
+    int64_t gd_offset;
     int64_t grain_offset;
     char filler[1];
     char check_bytes[4];