[BUG] vcsim: govc vm.disk.change to vcsim crashes by nil pointer dereference

[Syuparn]

Describe the bug

When govc vm.disk.change is requested to vcsim, govc panics and crashes by nil pointer dereference.

To Reproduce
Steps to reproduce the behavior:

1. run vcsim without any options
2. execute command below

# check disk in DC0_H0_VM0
$ govc device.info -vm DC0_H0_VM0 disk-*
Name:           disk-202-0
  Type:         VirtualDisk
  Label:        disk-202-0
  Summary:      10,485,760 KB
  Key:          204
  Controller:   pvscsi-202
  Unit number:  0
  File:         [LocalDS_0] DC0_H0_VM0/disk1.vmdk
# change the disk's size
$ govc vm.disk.change -vm DC0_H0_VM0 -disk.name disk-202-0 -size 10G

Expected behavior

govc raises no errors and vcsim increases the disk's size.

Affected version

• govc: 0.29.0
• vcsim: 0.29.0

Screenshots/Debug Output

• govc panics by nil pointer dereference as below

$ govc vm.disk.change -vm DC0_H0_VM0 -disk.name disk-202-0 -size 10G
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xd7ee28]

goroutine 1 [running]:
github.com/vmware/govmomi/govc/vm/disk.(*change).Run(0xc0001ee150, {0x13ddf80, 0xc000130000}, 0xe?)
        /home/runner/work/govmomi/govmomi/govc/vm/disk/change.go:159 +0x148
github.com/vmware/govmomi/govc/cli.Run({0xc00012e010, 0x7, 0x7})
        /home/runner/work/govmomi/govmomi/govc/cli/command.go:165 +0x32d
main.main()
        /home/runner/work/govmomi/govmomi/govc/main.go:114 +0x4f

Additional context
Add any other context about the problem here.

Panic is raised by this line.

https://github.com/vmware/govmomi/blob/master/govc/vm/disk/change.go#L159

Since vm.disk.change works with vCenter, this is due to vcsim models.

> govc device.info -vm vm1 "disk-*"
Name:           disk-1000-0
  Type:         VirtualDisk
  Label:        Hard disk 1
  Summary:      1,048,576 KB
  Key:          2000
  Controller:   pvscsi-1000
  Unit number:  0
  File:         [datastore1] vm1/vm1.vmdk
> govc vm.disk.change -vm vm1 "-disk.name" disk-1000-0 -size 10G
> govc device.info -vm vm1 "disk-*"
Name:           disk-1000-0
  Type:         VirtualDisk
  Label:        Hard disk 1
  Summary:      10,485,760 KB
  Key:          2000
  Controller:   pvscsi-1000
  Unit number:  0
  File:         [datastore1] vm1/vm1.vmdk