MAINTAINERS: cover tests/migration/s390x/

The generic s390 section looks like the best resting place.

Message-Id: <20190529091243.25562-1-cohuck@redhat.com>
Signed-off-by: NCornelia Huck <cohuck@redhat.com>

MAINTAINERS

@@ -102,6 +102,7 @@ F: pc-bios/s390-ccw/
 F: pc-bios/s390-ccw.img
 F: target/s390x/
 F: docs/vfio-ap.txt
+F: tests/migration/s390x/
 K: ^Subject:.*(?i)s390x?
 T: git https://github.com/cohuck/qemu.git s390-next
 L: qemu-s390x@nongnu.org