xen/xen_platform: QOM casting sweep

Define and use standard QOM cast macro. Remove usages of DO_UPCAST()
and direct -> style upcasting.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
[AF: Dropped opaque casts]
Signed-off-by: Andreas Färber <afaerber@suse.de>