s390/virtio: sort out physical vs virtual pointers usage

This does not fix a real bug, since virtual addresses
are currently indentical to physical ones.

Reviewed-by: Nico Boehr <nrb@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>