Expose vmi_init_arch to (re)initialize the arch interface

VMI_INIT_PARTIAL can now succeed in two ways: 1) giving access only to physical pages and vCPU registers 2) also allowing for v2p translation using dtb values. As during boot the page mode switches around, exposing arch reinit allows for keeping up with the latest page mode in-use without having to throw away the entire vmi instance to reinitialize everything. Once the final page mode is setup the user can switch to full mode as before with vmi_init_complete and derivatives (if required).