s390/kvm: Change the virtual memory mapping location for Virtio devices

The current location for mapping Virtio devices does not take
into consideration of the standby memory. This causes the failure
of mapping standby memory since the location for the mapping is
already taken by the Virtio devices. To fix the problem, we move
the location to beyond the end of standby memory.

Signed-off-by: Nick Wang <jfwang@us.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>