security_selinux: Use proper structure to access socket data
    
    In virSecuritySELinuxSetSecurityChardevLabel() we are labelling unix
    socket path, but accessing another structure of the union.  This does
    not pose a problem currently as both paths are at the same offset, but
    this should be fixed for the future.
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>