Description of problem:

If a RHEL8 (without ovirt-guest-agent) VM has the Memory Balloon device disabled (either manually or by the high performance profile:

1) In Admin Portal, Memory usage of Guest is always 0% as vdsm reports memUsage as 0.
2) memoryStats are missing for the Guest, all memory stats in the API are 0 too.

See:
# vdsm-client VM getStats vmID=e0bff6bc-0f64-4264-b036-bd122e4c5f38 | egrep 'memUsage|memoryStats'
        "memUsage": "0", 

It looks like VDSM first looks for Balloon stats, and then falls back to ovirt-guest-agent. But RHEL8 does not have oga.

https://github.com/oVirt/vdsm/blob/ovirt-4.3/lib/vdsm/virt/vm.py#L1799
https://github.com/oVirt/vdsm/blob/ovirt-4.3/lib/vdsm/virt/vmstats.py#L450

So without a balloon device this is not going to work.

Also found you tried to add this to qemu-ga in the past:
https://bugzilla.redhat.com/show_bug.cgi?id=1101915

Version-Release number of selected component (if applicable):
* vdsm-4.30.38-1.el7ev.x86_64
* RHEL8 
  * qemu-guest-agent-2.12.0-88.module+el8.1.0+5013+4f99814c.1.x86_64
  * no ovirt-guest-agent

How reproducible:
Always

Steps to Reproduce:
1. Install RHEL8 Guest
2. Disable balloon device (Edit -> Resource Allocation)
   Note: Or enabling High Performance
3. Start VM

Actual results:
Incorrect stats

Expected results:
Correct stats