In order to get devices appear in output of
"./qemu-system-ppc64 -device ?",
they must be assigned to one of DEVICE_CATEGORY_XXXX.

This puts VIO devices classes to corresponding categories.
Signed-off-by: NAlexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: NAlexander Graf <agraf@suse.de>