spice: avoid crashing when using invalid video config
    
    virt-viewer _only_ supports guests that have either:
    
    A) a signle graphics device with multiple displays (monitorid=0,
    displayid=(0,1,2,3)).
    
    B) multiple graphics device with a single display each
    (monitorid=(0,1,2,3), displayid=0).
    
    From now on, avoid crashing connecting to a guest which has a graphics
    configuration that violates A or B. However, even avoiding the crash, we
    cannot ensure the guest will work as expected.
    
    Resolves: rhbz#1250820
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>