[BUG] vcsim: virtualMachine.RootSnapshot is not updated when snapshot is created

[Syuparn]

Describe the bug

When a new snapshot is created for a virtual machine in vcsim, RootSnapshot is not updated and remains null.

To Reproduce
Steps to reproduce the behavior:

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

# create a new snapshot
$ govc snapshot.create -vm DC0_H0_VM0 my-snapshot
# check the snapshot is created
$ govc snapshot.tree -i -vm DC0_H0_VM0
[snapshot-67]  my-snapshot
[snapshot-67]  .
# show RootSnapshot
$ govc vm.info -json DC0_H0_VM0 | jq .VirtualMachines[].RootSnapshot

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

RootSnapshot contains a moid of the snapshot.

$ govc vm.info -json DC0_H0_VM0 | jq .VirtualMachines[].RootSnapshot
[
  {
    "Type": "VirtualMachineSnapshot",
    "Value": "snapshot-67"
  }
]

Affected version
Please provide details on the version used, e.g. release tag, commit, module version, etc.

• vcsim 0.29.0

Screenshots/Debug Output
If applicable, add screenshots or debug output to help explain your problem.

RootSnapshot is null.

$ govc vm.info -json DC0_H0_VM0 | jq .VirtualMachines[].RootSnapshot
null

Additional context
Add any other context about the problem here.

VirtualMachine.Snapshot.RootSnapshotList has root snapshot moids as expected.

$ govc vm.info -json DC0_H0_VM0 | jq .VirtualMachines[].Snapshot.RootSnapshotList[].Snapshot
{
  "Type": "VirtualMachineSnapshot",
  "Value": "snapshot-67"
}