IAllocator: some more info exported

This patch adds the following information to the exported info:
  - hypervisor type (in the main dict)
  - total memory used by primary instances (in each node dict) (can be
    computed from the node+instance dicts, but it's cheap to compute
    from Ganeti) (this is not live data, just from the config)
  - total memory used by primary instances marked to be up, similar to
    the above

Reviewed-by: imsnah