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>