As far as the commserver I think VM is just fine. I dont think you would run into any issue there. Media agents are another story. I assume you would want to build out VM media agents to take advantage of VMotion incase of a hardware failure. Some of the issue i would see.
1. Where would you house the DDB?
2. If you have a tape libary or tape drive mutile ESX host would need to be attached to it someway.
3. I am not sure Commvault fully supports VM media agents. I would be afraid to run into issue and Commvault comes back and says your media agent is Virtual and thats the problem
Have you ever tried to stand up another commserver and media agent as part of a DR recovery plan. We have in about 30 mintues with two different servers and IP addresses. I would suggest you practice this and put a DR plane together for commvault.
I would say yes to commserver for VM no for media agent. They require too much CPU and ram.