Fix checking of node free disk in CreateInstance

This patch does two things:
  - checks that the result values from call_node_info are valid integer
    values and aborts otherwise
  - skips disk space computation for the DT_DISKLESS case

The most important point of the patch is the verification of results
from the rpc call, as it prepares for a patch that allows failures to be
better reported from the remote node.

Reviewed-by: ultrotter