from LucD
$spec = New-Object VMware.Vim.VirtualMachineConfigSpec
$spec.extraConfig += New-Object VMware.Vim.OptionValue
$spec.extraConfig[0].key = "bios.bootDeviceClasses"
$spec.extraConfig[0].value = "allow:cd,hd"
(get-view (Get-VM -Name <VM-name>).ID).ReconfigVM_Task($spec)