Describe the bug

Since version v0.37.2, when trying to change a disk's size, I get the error :

$ ./govc vm.disk.change -vm VMNAME -disk.filePath '[DS] PATH/VM.vmdk' -size 29G
./govc: error resizing main disk
Logged Item:  Invalid operation for device '0'.

To Reproduce
Steps to reproduce the behavior:

1. Create a VM with a disk attached
2. Try to expand the disk using govc vm.disk.change -vm VMNAME -disk.filePath '[DS] PATH/VM.vmdk' -size XXG

Expected behavior
The disk should be updated with the new size without any error message, same as in v0.37.1

Affected version
Works in v0.37.1. It broke in v0.37.2