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=1421788Reported-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 1488789111-27340-1-git-send-email-kraxel@redhat.com

hw/display/qxl.c

@@ -477,6 +477,11 @@ static int qxl_track_command(PCIQXLDevice *qxl, struct QXLCommandExt *ext)
             qxl->guest_cursor = ext->cmd.data;
             qemu_mutex_unlock(&qxl->track_lock);
         }
+        if (cmd->type == QXL_CURSOR_HIDE) {
+            qemu_mutex_lock(&qxl->track_lock);
+            qxl->guest_cursor = 0;
+            qemu_mutex_unlock(&qxl->track_lock);
+        }
         break;
     }
     }