[BUG] vcsim: decreasing disk size should raise an error

[Syuparn]

Describe the bug

In ReconfigVM_Task, virtual disk size can be increased but not decreased.
However vcsim allows decreasing disk size.

To Reproduce

1. run vcsim without any options
2. check DC0_H0_VM0 has a disk whose size is 10GB
3. decrease the disk size by a script below

$ govc device.info -vm DC0_H0_VM0 disk-* | grep Summary
  Summary:      10,485,760 KB

vm := object.NewVirtualMachine(c.Client, types.ManagedObjectReference{
	Type:  "VirtualMachine",
	Value: "vm-55", // DC0_H0_VM0
})
devices, _ := vm.Device(ctx)
disk := devices.FindByKey(204).(*types.VirtualDisk)
// decrease capacity
disk.CapacityInBytes /= 2
disk.CapacityInKB /= 2

spec := types.VirtualMachineConfigSpec{
	DeviceChange: []types.BaseVirtualDeviceConfigSpec{
		&types.VirtualDeviceConfigSpec{
			Device:        disk,
			Operation:     types.VirtualDeviceConfigSpecOperationEdit,
			FileOperation: "",
		},
	},
}
task, _ := vm.Reconfigure(ctx, spec)

err := task.Wait(ctx)
fmt.Printf("error: %#v\n", err)

Expected behavior

vcsim responds InvalidDeviceOperation.

error: Invalid operation for device '0'.

Affected version

• vcsim 0.29.0

Screenshots/Debug Output

vcsim decreases the disk size without any errors.

error: <nil>

$ govc device.info -vm DC0_H0_VM0 disk-* | grep Summary
  Summary:      5,242,880 KB

Additional context
Add any other context about the problem here.

Even if capacity is not decreased, InvalidDeviceOperation is raised under following conditions.

• both CapacityInKB and CapacityInBytes are increased and
• both have diffrerent capacity

# neither CapacityInKB nor CapacityInBytes are modified: same as before (success)
> go run main.go
before:
kb: 2097156, bytes: 2147487744
modified spec:
kb: 2097156, bytes: 2147487744
after:
kb: 2097156, bytes: 2147487744

# only CapacityInKB is increased: new capacity is same as specified CapacityInKB (success)
> go run main.go
before:
kb: 2097156, bytes: 2147487744
modified spec:
kb: 2097157, bytes: 2147487744
after:
kb: 2097157, bytes: 2147488768

# only CapacityInBytes is increased: new capacity is same as specified CapacityInBytes (success)
> go run main.go
before:
kb: 2097157, bytes: 2147488768
modified spec:
kb: 2097157, bytes: 2147489792
after:
kb: 2097158, bytes: 2147489792

# both are increased: new capacity is same as specified (success)
> go run main.go
before:
kb: 2097158, bytes: 2147489792
modified spec:
kb: 2097159, bytes: 2147490816
after:
kb: 2097159, bytes: 2147490816

# both are increased but diffrerent values are specified (failure)
> go run main.go
before:
kb: 2097158, bytes: 2147489792
modified spec:
kb: 3097158, bytes: 2147490816
error: Invalid operation for device '0'.