KVM: s390: Helper for converting real addresses to absolute

Added a separate helper function that translates guest real addresses
to guest absolute addresses by applying the prefix of the guest CPU.

Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>