Add test for QA:Testcase_Anaconda_User_Interface_VNC
    
    This adds a test for QA:Testcase_Anaconda_User_Interface_VNC -
    the VNC install test case. It's implemented as a server/client
    pair, with the server booting from the Server DVD image with
    `inst.vnc` and the client booting from the desktop base disk
    image, setting up networking, then running Boxes to connect to
    the server and run the install.
    
    There are various little tweaks to test loading and logic to
    handle this, mostly pretty clear. We also move the workaround
    for 'spurious auth prompt appears on desktop after you switch
    away to a VT and back' out of the desktop update test and into
    the `desktop_vt` helper function, since now this test can hit
    it as well. We enhance _graphical_wait_login to handle the boot
    loader if needed (it has never needed to until now).
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>