MAINTAINERS: Replace backup for s390 vfio-pci

MAINTAINERS: Replace backup for s390 vfio-pci

Farhan has been doing a masterful job coming on in the
s390 PCI space, and my own attention has been lacking.
Let's make MAINTAINERS reflect reality.

Signed-off-by: Eric Farman <farman@linux.ibm.com>
Acked-by: Alex Williamson <alex@shazbot.org>
Acked-by: Farhan Ali <alifm@linux.ibm.com>
Acked-by: Matthew Rosato <mjrosato@linux.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>