test_po_cleanup is unstable on KVM

[daall]

Description

Steps to reproduce the issue:

1. Open a PR
2. Run the test_po_cleanup test

Describe the results you received:
Test fails frequently. See:

https://sonic-jenkins.westus2.cloudapp.azure.com/job/vs/job/buildimage-vs-image-pr/test_results_analyzer/

https://sonic-jenkins.westus2.cloudapp.azure.com/job/common/job/sonic-mgmt-testing/job/sonic-mgmt-pr/test_results_analyzer/

#5133 (comment)

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)
```

[daall]

@yxieca FYI

[abdosi]

Looking into this.

[lguohan]

https://sonic-jenkins.westus2.cloudapp.azure.com/job/common/job/sonic-mgmt-testing/job/sonic-mgmt-pr/1429/testReport/junit/pc/test_po_cleanup/test_po_cleanup/

looks like we still consistently see this issue.

[abdosi]

@lguohan Sonic-swss submodule update PR is still not merged.

#5489