A snippet from collaboration to develop a spec that you can feed to autoStartManager

auto_power_on.py

        testvm = # handwaving
        spec = host.configManager.autoStartManager.config

        auto_power_info = vim.host.AutoStartManager.AutoPowerInfo()
        auto_power_info.key = testvm
        auto_power_info.startAction = vim.VirtualMachine.PowerState.poweredOn #note use of constant instead of string
        auto_power_info.startDelay = 60
        auto_power_info.startOrder = 1
        auto_power_info.stopAction = None # note: the Python constant 'None' not a string
        auto_power_info.stopDelay = -1 # note the use of numeric value here.

        spec.powerInfo = [auto_power_info]

        host.configManager.autoStartManager.ReconfigureAutostart(spec)