Missing support for mixed multiple monitors

Hi, looking at #13 (closed) and #8 (closed) and at agent code if you have a mix of QXL and other monitors mouse is not working. This as in this case the detected displays (DesktopLayout::_displays attribute) is empty and mouse events are ignored. It seems that server assume that if there's an agent in the guest it always can handle mouse events so the agent is not able to tell the server to use server mouse mode. See commit 6b74ffb8.