[core] Make `ruby-libvirt` dependency optional

The `ruby-libvirt` dependency was commented out since it is rarely used
and a pain for developers to prepare.

The comment was removed in https://github.com/fog/fog/commit/b373e55a8f32b24c11fe4b20e773c1428df9d811
to protect JRuby but made it a requirement for MRI (& others).

This adds another guard based around the setting of `FOG_USE_LIBVIRT`
environment variable.

This hopefully will mean anyone wanting to use it can just set that
rather than editing the gemspec.

Fixes #2316