libvmi core: call get_memory_layout() during partial init 

In existing code, paging mode and cr3 are not probed until full initialization. However, get_memory_layout() really only probes register values, so it should be safe to run during partial initialization (the function already exits as a NOP when the target is a file, rather than a live VM).  This small change opens up a variety of functions for use prior to full initialization which would otherwise be unavailable.