Feature #1739: Move the logic of what to do with a failed poll to VMMDriver
Feature #1739: Drivers inform that the VM_POLL info is included, making things easier for the core when 0 vms are found
Feature #1739: Monitor disabled hosts also
Feature #1739: Improve host monitoring performance

We don't need to retrieve and parse each Host xml body column, we
already have the Hosts in the cache