KVM: s390: update maintainers

This patch adds an entry for kvm on s390 to the MAINTAINERS file :-). We intend
to push all patches regarding this via Avi's kvm.git.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Carsten Otte <cotte@de.ibm.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>