MAINTAINERS: Add myself as virtio-mem maintainer

Let's make sure patches/bug reports find the right person.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20200507140139.17083-3-david@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>