qxl: clear guest_cursor on QXL_CURSOR_HIDE

Make sure we don't leave guest_cursor pointing into nowhere.  This might
lead to (rare) live migration failures, due to target trying to restore
the cursor from the stale pointer.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1421788
Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 1488789111-27340-1-git-send-email-kraxel@redhat.com
