test_po_update flaky on KVM

[daall]

Description
Following #5231 the test_po_update became extremely flaky on the PR runners. It is still passing intermittently so we're not sure if this is a test issue or an image issue introduced by the submodule update.

Steps to reproduce the issue:

1. Run test_po_update on a virtual switch w/ the latest master image.

Describe the results you received:
Test fails most of the time.

Describe the results you expected:
Test should pass consistently.

Additional information you deem important (e.g. issue happens only occasionally):

**Output of `show version`:**

```
(paste your output here)
```

**Attach debug file `sudo generate_dump`:**

```
(paste your output here)
```

[anshuv-mfst]

@yxieca to follow-up on this issue.

[daall]

This issue is also seen on physical switches running the master image.