MAINTAINERS: Add myself as virtio-balloon co-maintainer

MAINTAINERS: Add myself as virtio-balloon co-maintainer

As suggested by Michael, let's add me as co-maintainer of virtio-balloon.
While at it, also add "balloon.c" and "include/sysemu/balloon.h" to the
file list.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20200312133725.8192-1-david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>