test(vm): filter only virt-launcher pods in volume migration test

[yaroslavborbat]

Description

Add label selector to filter only virt-launcher pods when checking pod count during VM volume migration test.
Previously, all pods in the namespace were listed, including hotplug volume (hp) pods, which caused incorrect count assertions.

Why do we need it, and what problem does it solve?

During the volume migration e2e test, the code checks that exactly 2 pods exist (source and target virt-launcher).
Without filtering, hotplug volume pods are also counted, leading to flaky test failures when the actual virt-launcher pod count is correct but total pod count differs.

What is the expected result?

1. Trigger VM volume migration
2. Verify that only virt-launcher pods are counted during migration
3. Test passes consistently without flakiness caused by hotplug pods

Checklist

• The code is covered by unit tests.
• e2e tests passed.
• Documentation updated according to the changes.
• Changes were tested in the Kubernetes cluster manually.

Changelog entries

section: test
type: chore
summary: Filter only virt-launcher pods in volume migration e2e test to avoid flakiness caused by hotplug volume pods.
impact_level: low