MAINTAINERS: Add vfio-ccw maintainer

Add Cornelia Huck as the vfio-ccw maintainer.

Acked-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Message-Id: <20170517004813.58227-14-bjsdjshi@linux.vnet.ibm.com>
[CH: add tree]
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>