vdpa: Make use of PFN_PHYS/PFN_UP/PFN_DOWN helper macro

it's a nice refactor to make use of
PFN_PHYS/PFN_UP/PFN_DOWN helper macro

Signed-off-by: Cai Huoqing <caihuoqing@baidu.com>
Link: https://lore.kernel.org/r/20210802013717.851-1-caihuoqing@baidu.com


Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>