Allow specifying `basefolder` when creating virtualbox VM

What does this MR do?

Allows specifying the base folder in which to create new virtualbox VMs

Why was this MR needed?

In our setup, we store the VMs across multiple SSDs to avoid bottlenecking when there are a large number of jobs running simultaneously. This allows us to automatically split the VMs across the drives by specifying a different target location for each base image.

The clonevm command already supports the --basefolder flag to achieve this, this MR simply adds the ability to configure this flag from the config.toml