Support clock=variable relative to localtime
    
    Since Xen 3.1 the clock=variable semantic is supported. In addition to
    qemu/kvm Xen also knows about a variant where the offset is relative to
    'localtime' instead of 'utc'.
    
    Extends the libvirt structure with a flag 'basis' to specify, if the
    offset is relative to 'localtime' or 'utc'.
    
    Extends the libvirt structure with a flag 'reset' to force the reset
    behaviour of 'localtime' and 'utc'; this is needed for backward
    compatibility with previous versions of libvirt, since they report
    incorrect XML.
    
    Adapt the only user 'qemu' to the new name.
    Extend the RelaxNG schema accordingly.
    Document the new 'basis' attribute in the HTML documentation.
    Adapt test for the new attribute.
    
    Signed-off-by: Philipp Hahn <hahn@univention.de>