conf: Validate disk lun using correct types
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1201143
    
    The formatdomain.html description for <disk> device 'lun' indicates that
    it must be either a type 'block' or type 'network' with protocol 'iscsi';
    however, we did not make that check until domain startup.
    
    This caused issues for virt-manager which had an unexpected failure at
    run time rather config time.
    
    This patch adds a check in post part disk device checking for the specific
    and supported lun types as well as adjusting the test failure to be for
    parse config rather than run time.