Currently the batcher hypervisor parameter must be a dict with one
element (e.g. {"xen-hvm": { "acpi": true }}). This is overly complex and
hard to validate correctly; the patch splits it in two:
  - one "hypervisor" string parameter, with the name of the hypervisor
  - one "hvparams" dictionary, with the hypervisor parameters

The patch also changes the error handling in parsing the definition file
- since this is not a long-running file, we are less concerned with safe
closing of the file, and more with presenting meaningful error
messages.

Reviewed-by: killerfoxi