Refactor xenapi/host.py to new call_xenapi pattern

Update all calls to call_xenapi to the new calling pattern.

This code calls out to the "host" object that this change adding into
the collection of session objects.

An expensive call to host.get_record has been changed to the cheaper
call of get_uuid.

Part of blueprint xenapi-driver-refactor
Change-Id: I4bc4e3f990e36d422beee2a7a9f5fa413044052a
Merge "Refactor xenapi/host.py to new call_xenapi pattern"