KVM: s390: Change maintainer

Since Carsten is now working on a different project, Cornelia will
work as the 2nd s390/kvm maintainer.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
CC: Carsten Otte <cotte@de.ibm.com>
CC: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>