Misc XML schema fixes for libxl
    
    The libxlxml2domconfigdata directory was not covered in the RNG schema
    tests. This hid a few bugs in both the libxl XML files and the RNG
    schema itself.
    
    Reviewed-by: Jim Fehlig <jfehlig@suse.com>
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>