[FEATURE] Roll out agents with new images after upgrading the manager

[starbops]

Is your feature request related to a problem? Please describe.

Existing agents will remain if the user upgrades virt-controller with an image other than the previous one. For instance, virt-controller was previously deployed with v0.3.1 but then upgraded to v0.3.2; existing agent pods still run in v0.3.1.

Describe the solution you'd like

This should require no human intervention. After successfully upgrading the manager, it should automatically roll out new agents. We already expose an option for users to specify what image to use for agents by the manager command, so let's leverage that.

Describe alternatives you've considered

Delete the agent pods forcefully after the manager is upgraded and let the reconciliation do its job.

Additional context

N/A