util: add APIs for reading/writing from/to rotating files
    
    Add virRotatingFileReader and virRotatingFileWriter objects
    which allow reading & writing from/to files with automation
    rotation to N backup files when a size limit is reached. This
    is useful for guest logging when a guaranteed finite size
    limit is required. Use of external tools like logrotate is
    inadequate since it leaves the possibility for guest to DOS
    the host in between invokations of logrotate.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>