drm: update MAINTAINERS for qemu drivers (bochs, cirrus, qxl, virtio-gpu)

Changes:
 * add myself as maintainer, so patches land in my inbox.
 * add virtualization@lists.linux-foundation.org mailing list.
 * add drm-qemu git repo.
 * flip bochs and qxl status to "Maintained".

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>