[BUG] vcsim: change VirtualDisk size decreases datastores freespace even if the size is same as before

[Syuparn]

Describe the bug

Changing VirtualDisk size by ReconfigVM_Task decreases datastore's freespace even if the size is same as before.

To Reproduce

1. run vcsim without any options
2. check datatstore's freespace
3. resize capacity of a disk (size is same as before)
4. check datatstore's freespace again

# datastore freespace is 10,200GB
$ govc datastore.info
Name:        LocalDS_0
  Path:      /DC0/datastore/LocalDS_0
  Type:      OTHER
  URL:       /tmp/govcsim-DC0-LocalDS_0-1270584146
  Capacity:  10240.0 GB
  Free:      10200.0 GB

# disk size is 10GB
$ govc device.info -vm DC0_H0_VM0 disk-*
Name:           disk-202-0
  Type:         VirtualDisk
  Label:        disk-202-0
  Summary:      10,485,760 KB
  Key:          255511522
  Controller:   pvscsi-202
  Unit number:  0
  File:         [LocalDS_0] DC0_H0_VM0/disk1.vmdk

# change disk size to 10GB (= same size)
$ govc vm.disk.change -vm DC0_H0_VM0 -size 10G
$ govc datastore.info

Expected behavior
A clear and concise description of what you expected to happen.

Datastore has same freespace as before.

$ govc datastore.info
Name:        LocalDS_0
  Path:      /DC0/datastore/LocalDS_0
  Type:      OTHER
  URL:       /tmp/govcsim-DC0-LocalDS_0-1270584146
  Capacity:  10240.0 GB
  Free:      10200.0 GB

Affected version

• vcsim 0.28.0

Screenshots/Debug Output

Datastore freespace decreases 10GB.

$ govc datastore.info
Name:        LocalDS_0
  Path:      /DC0/datastore/LocalDS_0
  Type:      OTHER
  URL:       /tmp/govcsim-DC0-LocalDS_0-1270584146
  Capacity:  10240.0 GB
  Free:      10190.0 GB

Additional context
Add any other context about the problem here.

Removing disk increases datastore freespace as expected.

$ govc datastore.info
Name:        LocalDS_0
  Path:      /DC0/datastore/LocalDS_0
  Type:      OTHER
  URL:       /tmp/govcsim-DC0-LocalDS_0-1270584146
  Capacity:  10240.0 GB
  Free:      10200.0 GB

$ govc vm.disk.create -vm DC0_H0_VM0 -name DC0_H0_VM0/disk2 -size 10G
[05-07-22 21:08:20] Creating disk

$ govc datastore.info
Name:        LocalDS_0
  Path:      /DC0/datastore/LocalDS_0
  Type:      OTHER
  URL:       /tmp/govcsim-DC0-LocalDS_0-1270584146
  Capacity:  10240.0 GB
  Free:      10190.0 GB

$ govc device.remove -vm DC0_H0_VM0 disk-202-1

$ govc datastore.info
Name:        LocalDS_0
  Path:      /DC0/datastore/LocalDS_0
  Type:      OTHER
  URL:       /tmp/govcsim-DC0-LocalDS_0-1270584146
  Capacity:  10240.0 GB
  Free:      10200.0 GB