I hope this is a lightly used server and does not do any real level of mail traffic else you'll soon regret running in any VM :)
Don't mean to start a flame war, but your statement above is just simply inaccurate. The main difference between a virtual server and a physical server is that resources are contended with other VMs in the virtual server suitation. So for example, if Dovecot required a lot of resources, and there were another 1000 idle VMs on the same physical box as the Dovecot VM, Dovecot would run fine. What is true for any service (not just Dovecot) is that you need to get the balance right depending on your server resources requirements and sharing the physical resources with other VMs in such a way that you don't starve some critical services.
So please stop with the FUD about virtualisation :)
BTW, I'm assuming a proper virtualisation solution such as Xen Paravirtualisation.