VMI_INIT_PARTIAL Fixes

1. `*_va()` methods should return VMI_FAILURE.
2. Should be supported for live VMs that don't have paging enabled.
This method is called `probe_memory_layout`, which it did successfully.  It just successfully determined that paging is not enabled.  All the other cases that return failures are serious, unrecoverable errors.

`arch_init()` fails because it does not support VM_PM_UNKNOWN, which is why I think it makes the code more understandable to set `ret = VMI_FAILURE` in that case.