htools: add more IAlloc data types and functions

These move the 'not supported' message deeper into the workflow;
assuming that the workflow as added by this patch is correct, all that
remains is:

- for disk template DRBD8, plug in the currently existing mechanisms
  to implement relocation
- for other disk templates, first modify the Node function to handle
  node changes correctly for each disk template, and then add support
  for them in nodeEvacInstance

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>