htools: add a function for computing evacuated nodes

The new IAllocator interface is based on instances, not nodes, so we
need to backtrack and compute on which nodes we can't allocate
instances during the current operation. This patch adds a function for
computing this node set.

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