qemu breaks vmdk larger than 600GB.
Description of problem:
The vmdk larger than 600G is corrupted after an edit by qemu-nbd. If I open the corrupted vmdk file, I find an extra **^@** byte.
```
RW 4194304 SPARSE "disk-s289.vmdk"
RW 4^@94304 SPARSE "disk-s290.vmdk"
RW 4194304 SPARSE "disk-s291.vmdk"
```
Steps to reproduce:
```
   qemu-img create -f vmdk -o subformat=twoGbMaxExtentSparse disk.vmdk 1T
   sudo qemu-nbd -c /dev/nbd0 disk.vmdk
   sudo mkfs.btrfs /dev/nbd0
   sudo qemu-nbd -d /dev/nbd0
   qemu-img info disk.vmdk | head
   ```
Additional information: