VMs with qemu guest agent don't render utilization disk chart

[sjd78]

For a VM with qemu guest agent, disk file system data may not be reported and the utilization graph is empty.

VM Portal version number: 1.5.2-1 (and 1.5.3-0 / master)
oVirt version number: 4.3 and 4.4
Browser and version: Firefox and Chrome, any version

Steps to Reproduce:

1. Have a VM with the qemu guest agent installed
2. Open VM details
3. Witness no Disk utilization chart

Screen shot:

VM's disks.usage statistics:

{
  "kind": "gauge",
  "type": "string",
  "unit": "none",
  "values": {
    "value": [
      {
        "detail": "[]"
      }
    ]
  },
  "vm": {
    "href": "/ovirt-engine/api/vms/10ebb6ac-645d-42ec-8031-c844aa14666a",
    "id": "10ebb6ac-645d-42ec-8031-c844aa14666a"
  },
  "name": "disks.usage",
  "description": "Disk usage, in bytes, per filesystem as JSON (agent)",
  "href": "/ovirt-engine/api/vms/10ebb6ac-645d-42ec-8031-c844aa14666a/statistics/c5dd0086-d5f0-3abf-9628-4674abb3f5ac",
  "id": "c5dd0086-d5f0-3abf-9628-4674abb3f5ac"
}

Guest VM info:

[sdickers@centos7_1 ~]$ cat /etc/system-release
CentOS Linux release 7.6.1810 (Core) 
[sdickers@centos7_1 ~]$ yum info qemu-guest-agent
Loaded plugins: fastestmirror
Repodata is over 2 weeks old. Install yum-cron? Or run: yum makecache fast
Loading mirror speeds from cached hostfile
 * base: ewr.edge.kernel.org
 * extras: mirror.clarkson.edu
 * updates: mirror.clarkson.edu
Installed Packages
Name        : qemu-guest-agent
Arch        : x86_64
Epoch       : 10
Version     : 2.12.0
Release     : 2.el7
Size        : 293 k
Repo        : installed
From repo   : base
Summary     : QEMU guest agent
URL         : http://www.qemu.org/
License     : GPLv2
Description : qemu-kvm is an open source virtualizer that provides hardware emulation for
            : the KVM hypervisor.
            : 
            : This package provides an agent to run inside guests, which communicates
            : with the host over a virtio-serial channel named "org.qemu.guest_agent.0"
            : 
            : This package does not need to be installed on the host OS.

[sdickers@centos7_1 ~]$