IRP_MN_QUERY_DEVICE_RELATIONS is called for each PDO you get this at boot
time, whenever you do scan for new hardware wizard and with
IoInvalidateDeviceRelations(). Now for a bus driver, see the toaster bus
sample, you create the PDOs of the children and return the array of PDOs
that the parent supports. This is then done recursively with each PDO you
report so that you can have an heirarchical array of parent and children.
Next AddDevice is called on each PDO where you create the FDO and do
AttachDevicetoDeviceStack().