Description of problem:
Trying to set the Custom Compatibility Version of a VM to an empty value via REST api (in order to take the Custom Compatibility Version value of the Cluster) is not supported, 

This should be done by running the following:
POST /ovirt-engine/api/vms/<vmid>
<vm>
    <custom_compatibility_version/>
</vm>

but it fails with an error: 
<detail>
For correct usage, see: http://butterfly.usersys.redhat.com:8080/ovirt-engine/api/v4/model#services/vm-graphics-consoles/methods/add
</detail>
<reason>Request syntactically incorrect.</reason>

Version-Release number of selected component (if applicable):
4.1 master

How reproducible:
100%

Steps to Reproduce:
1.Try to create a vm or edit a vm and set custom_compatibility_version attribute to an empty value:
<vm>
    <custom_comaptibility_version/>
</vm>

Actual results:
It failed with error:
<detail>
For correct usage, see: http://butterfly.usersys.redhat.com:8080/ovirt-engine/api/v4/model#services/vm-graphics-consoles/methods/add
</detail>
<reason>Request syntactically incorrect.</reason>

Expected results:
It should succeeded.