Description of problem:
"datum" child node for "value" node appears to be missing in some responses from RestAPI while it is mandatory on schema.xsd

From: /api/hosts/3f978460-407a-48e9-ae9d-2ad40e3ce59e/nics/270ad0a2-2739-4ef2-90f3-d424e1cb3e07/statistics
...
    <statistic href="/api/hosts/3f978460-407a-48e9-ae9d-2ad40e3ce59e/nics/270ad0a2-2739-4ef2-90f3-d424e1cb3e07/statistics/0b9ddd6a-fc1b-398a-9f95-7a863c753017" id="0b9ddd6a-fc1b-398a-9f95-7a863c753017">
        <name>data.total.rx</name>
        <description>Total received data</description>
        <values type="INTEGER">
            <value/>
        </values>
        <type>COUNTER</type>
        <unit>BYTES</unit>
        <host_nic href="/api/hosts/3f978460-407a-48e9-ae9d-2ad40e3ce59e/nics/270ad0a2-2739-4ef2-90f3-d424e1cb3e07" id="270ad0a2-2739-4ef2-90f3-d424e1cb3e07"/>
    </statistic>
...

From ovirt-engine/api?schema
...
      <xs:element name="datum" type="xs:decimal" minOccurs="1" maxOccurs="1"/>
...


Version-Release number of selected component (if applicable):
rhevm-3.6.9.1-0.1.1.el6.noarch

How reproducible:
100%

Steps to Reproduce:
1. Query rest api for /api/vms/{id}/nics/{id} endpoint on a new VM
2. Check datum node is present under value node

Actual results:
datum field missing

Expected results:
datum field is returned and RestAPI response is validated against schema.xsd

Additional info:
Also found under:
/api/vms/{id}/nics/{id}