Return to the vanilla kernel

Our choice of the Zen kernel was a bit random and speculative. So far we have a report that it hurts something, and no reports that it helps anything.

Over time the differences between the Zen kernel and the vanilla one have dwindled, and I can barely find an up-to-date list of the differences without looking through source code.

I would consider the upstream vanilla sources to be safer and more trustworthy, and I expect will be much easier to get through shim review. Let's return to them.

This also adds vhba-module as it is required for cdemu-client; apparently it was built into the Zen kernel, but it's separate for the vanilla one.

Resolves #601 (closed)