remote-viewer: Prefer ca-cert from display instead of proxy
    
    Since oVirt engine version 4.3.2.1, the API returns certificate data for
    display connection in the VM XML, so users do not need to specify it
    from the command line anymore. The certificate obtained from the XML
    gets precedence over the one from the command line, which is still kept
    to keep compatibility of older versions of oVirt.
    
    Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1402909
    
    Signed-off-by: Eduardo Lima (Etrunko) <etrunko@redhat.com>