XenAPI: Fix config section usage

Change I2e9f720ddce284fc112d6a5651fd277e6e31a17a introduced xenserver
sections but a couple of CONF variables were not updated to use the
new sections.

Fixed the few remaining options and add unit tests to ensure they are
caught in future.

The unit tests highlighted that the functions behaved strangely in
the cases where the default was being provided - so updated to be
clear about getting the default.

Closes-bug: 1253064

Change-Id: Ic538069cd087aa3cdc0a1f7fdb94820d5d3b22b1
Merge "XenAPI: Fix config section usage"