qxl: remove qxl_io_log()

qxl_io_log() sends messages over to the host (qemu) for logging.
Remove the function and all callers, we can just use standard
DRM_DEBUG calls (and if needed a serial console).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20180420071904.24276-2-kraxel@redhat.com