misc/ivshmem: QOM Upcast 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: Simplify casts]
Signed-off-by: Andreas Färber <afaerber@suse.de>