[Vsphere] Isolate helper from core

`Fog.class_from_string` was moved to core but is only used by the
VSphere provider with very little capacity for reuse.

This moves it to the `Fog::Vsphere` namespace so we can delete it from
core.
Merge pull request #2787 from fog/isolate_vsphere_helper

[Vsphere] Isolate helper from core