conf: add device pointer to args of virDomainDeviceInfoIterate callback

conf: add device pointer to args of virDomainDeviceInfoIterate callback

There will be cases where the iterator callback will need to know the
type of the device whose info is being operated on, and possibly even
need to use some of the device's config. This patch adds a
virDomainDeviceDefPtr to the args of every callback, and fills it in
appropriately as the devices are iterated through.