Use server_id for Vsphere::Interfaces and Vsphere::Volumes

Attempt to resolve the infinite loop when calling <code><Vsphere::Server>.inspect</code>. The loop is introduced because each <code><Vsphere::Server></code> instance has a reference to the <code><Vsphere::Interfaces></code> and <code><Vsphere::Volumes</code> collections. Each of these collections, in turn, has an attribute <code>server</code>, which refers to the <code><Vsphere::Server></code> object. This circular dependency breaks the _Fog_ <inspect()> method, which attempts to print out each of the attributes of a given object.

The proposed resolution is to introduce a functional change, modifying the aforementioned models to expect the ID of the <code><Vsphere::Server></code>. The models then lookup the server object as necessary.

Fixes: #2938