The root device object is in fact a PDO and a FDO at the same time. Thus
there is no need in creating two device objects here, one is enough.
This commit also removes the explicit device extension for the root DO,
because the only reason it existed is to distinguish the root driver's
FDO from its PDOs. This can easily be done by comparing with
IopRootDeviceNode.
Also collect some unused garbage while we are here.
Handling PnP root driver power IRPs requires that a device object must come up
with a device extension to determine whether it is a function driver and if so,
handle the IRP accordingly.
CORE-18989
This also makes it necessary to fix a bug in the previous code:
ZwEnumerateKey will not account for space for a null terminator, so to
ensure we have space, we must allocate the additional WCHAR, but not
include it in the buffer size passed to the function.
