Thank to user xyz222 and the post Re: Create VM instance on Cluster
I was able to fin the problem. So simple... The value of the <Guest-Id> in the xml file, has to be for example "rhel6_64Guest" and not a random word... that is it!
sudo ./vmcreate.pl --server SERVER --username LOGIN --password PASS --filename ../sampledata/vmcreate.xml --schema ../schema/vmcreate.xsd
Successfully created virtual machine: 'teeest2' under host HOST_NAME