Description of problem:
Now statistics response for disk.usage of not running VM is <values><value><detail></detail></value></values>.
for other statistics, if there is a no value, it appears, like  <values/> or
        <values>
            <value>
                <datum>0</datum>
            </value>
        </values>

Version-Release number of selected component (if applicable):4.4.3

How reproducible:always


Steps to Reproduce:
1. Create VM on the base of last infra template (tried both latest-rhel-guest-image-8.1-infra and 8.2).
2. Send Get statistics request for the VM (not running)
https://{{host}}/ovirt-engine/api/vms/{{vmID}}/statistics.

Actual result:
   <statistic href="/ovirt-engine/api/vms/38529667-032a-4655-a22f-a17ff83f5dc8/statistics/c5dd0086-d5f0-3abf-9628-4674abb3f5ac" id="c5dd0086-d5f0-3abf-9628-4674abb3f5ac">
        <name>disks.usage</name>
        <description>Disk usage, in bytes, per filesystem as JSON (agent)</description>
        <kind>gauge</kind>
        <type>string</type>
        <unit>none</unit>
        <values>
            <value>
                <detail></detail>
            </value>
        </values>
        <vm href="/ovirt-engine/api/vms/38529667-032a-4655-a22f-a17ff83f5dc8" id="38529667-032a-4655-a22f-a17ff83f5dc8"/>
    </statistic>

Expected results:
for other statistics, if there is a no value, it appears, like  <values/> or
        <values>
            <value>
                <datum>0</datum>
            </value>
        </values>

also <unit>none</unit> must be changed to <unit>bytes</unit>

Additional info:
related to https://bugzilla.redhat.com/show_bug.cgi?id=1814565