NMExportedObject is the center of every D-Bus exported object in
NetworkManager's core. It makes sense to optimize it.

Transform the GSList of interfaces to be a array. The array is still
allocated via the slice allocator (as we expect that there are only few
types in the list). This saves the overhead to allocate a GSList item
for each entry.

Another advantage is that the interfaces list is now strongly typed
instead of an opaque data pointer.