Add common API for doing polkit authentication
    
    There are now two places in libvirt which use polkit. Currently
    they use pkexec, which is set to be replaced by direct DBus API
    calls. Add a common API which they will both be able to use for
    this purpose.
    
    No tests are added at this time, since the impl will be gutted
    in favour of a DBus API call shortly.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>