[S390] kvm_s390: Fix oops in virtio device detection with "mem="

The current virtio model on s390 has the descriptor page above the main
memory. The guest virtio detection will oops if the mem= parameter is
used to reduce/change the memory size.
We have to use real_memory_size instead of max_pfn to detect the virtio
descriptor pages.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>