KVM provider: restructured to always use Fog.io to connect to libvirt

As discussed in pull request #465, I restructured the KVM provider to always use Fog.io instead of a direct connection to libvirt. This allows to configure the libvirt parameters in the fog.io config file and should fix issue #470.

I also extended the documentation for KVM with some more details on prerequisites and usage of the KVM provider.

I successfully tested this pull request, but would welcome feedback by others if it is working fine.