Here's a working snippet:

vm = api.vms.get(name=vm_name)

action = params.Action(
    use_cloud_init=True,
    vm=params.VM(
      initialization=params.Initialization(
        regenerate_ssh_keys=True,
        host_name=vm_fqdn,
        nic_configurations=params.GuestNicsConfiguration(
          nic_configuration=[
            params.GuestNicConfiguration(
              name="eth0",
              boot_protocol="static",
              on_boot=True,
              ip=params.IP(
                address=vm_address,
                netmask=vm_netmask,
                gateway=vm_gateway,
                ),
              ),
            ],
          ),
        ),
      ),
    )

vm.start(action)