MAINTAINERS: Add entry for s390 vfio-pci

Add myself to cover s390-specific items related to vfio-pci.

Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>