qxl: fix local renderer crash

qxl: fix local renderer crash

Make sure we only ask the spice local renderer for display updates in
case we have a valid primary surface.  Without that spice is confused
and throws errors in case a display update request (triggered by
screendump for example) happens in parallel to a mode switch and hits
the race window where the old primary surface is gone and the new isn't
establisted yet.

Cc: qemu-stable@nongnu.org
Fixes: https://bugzilla.redhat.com//show_bug.cgi?id=1567733
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20180427115528.345-1-kraxel@redhat.com