Description of problem:
In 3.6:

params.OperatingSystem(
    type_='unassigned',
    boot=(
        params.Boot(dev='cdrom'),
        params.Boot(dev='hd'),
    ),
)

now translates to

params.OperatingSystem(
    type_='unassigned',
    boot=params.Boot(
        devices=params.devicesType(
            device=[
                'cdrom',
                'hd',
            ],
       ),
    )
)

while I would have expected something like:

params.OperatingSystem(
    type_='unassigned',
    boot=params.Boot(
        devices=['cdrom', 'hd'],
    )
)

to just store a list of strings.