Editor got rid of an empty line with a tab on it
Check parent keys exist for node#lxc? node#vmware?

This adds an additional helper method to validate if node['virtualization']['system'] even exists. It will emit a Chef::Log.warn() about the ohai virtualization plugin not being loaded if it finds attributes are just entirely missing. This will prevent `undefined method '[]' for nil:NilClass` exceptions.

I've also adjusted the tests to check for actual false, not just falsey, since these methods no longer will return `nil`.

Finally, there was an empty line with a tab on it that my editor wanted to correct.