Try:
$installerArgs = 'REBOOT="ReallySuppress"'
(Get-VM | Get-View).UpgradeTools_Task($installerArgs)
Regards Robert
Blog: https://rvdnieuwendijk.com/ | Twitter: @rvdnieuwendijk | Author of: https://www.packtpub.com/virtualization-and-cloud/learning-powercli-second-edition