IMedium#createBaseStorage takes a list of MediumVariant as second argument.

Hi!

I found out that the method IMedium#createBaseStorage takes a list of MediumVariants instead of a flag as second argument.
In this commit, there is a fix for the function `vmfest.virtualbox.image/create-base-storage`. For meaningful logging, I built a vector of MediumVariants.

Cheers,
Stan.