Используя плагин POJO Generator в Android Studio, как рекомендовано здесь , я узнал, как сделать мои классы данных
@Root
data class Content(@field:Element(name = "Items", required = false) var items: Items? = null)
data class Items(
@field:ElementList(name = "Device", required = false, entry = "Device", inline = true)
var deviceList: List<Device>? = null,
@field:Attribute(name="count", required = false)
var count: Int? = null
)
data class Device(@field:Element(name="User", required = false) var user: User? = null)
data class User(@field:Element(name="Lastname", required = false) var lastname: String? = null,
@field:Element(name="Firstname", required = false) var firstname: String? = null)