Nice approach can be found
here.
# WMI Query that specifies what events we will watch for
$Query = "Select * from __InstanceModificationEvent within 3 where TargetInstance ISA 'MSVM_ComputerSystem' `
and TargetInstance.EnabledState <> PreviousInstance.EnabledState"
# Script block that will run whenever the event fires
[ScriptBlock]$Action = {
$VMName = $Event.SourceEventArgs.NewEvent.TargetInstance.ElementName
switch ($Event.SourceEventArgs.NewEvent.TargetInstance.EnabledState)
{
2 {$vmState = "running"}
3 {$vmState = "turned off"}
9 {$vmState = "paused"}
6 {$vmState = "in a saved state"}
10 {$vmState = "starting"}
4 {$vmState = "stopping"}
default {$vmState = "in an unknown state..."}
}
if ($Event.SourceEventArgs.NewEvent.TargetInstance.EnabledState -eq 1)
{$vmState = $Event.SourceEventArgs.NewEvent.TargetInstance.OtherEnabledState}
write-host "The virtual machine '$($vmName)' is now $($vmState)."}
# Register for the events
Register-WMIEvent -Query $Query -Action $Action -Namespace root\virtualization\v2
# To clean up - run "Get-EventSubscriber | Unregister-Event"