drm/qxl: add spice-devel list to MAINTAINERS

So qxl kernel patches are sent to the spice-devel list for review.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20181121090129.23506-1-kraxel@redhat.com