Move xenapi/agent.py to new call_xenapi style

At the same time, we used to call get_record to just fetch the domid,
this is very expensive compared to just call get_domid, so I have moved
towards using the cheaper call, particularly because this is called many
times when booting a server.

Part of blueprint xenapi-driver-refactor
Change-Id: If489f41c3d90ba9f738cd56bbc1277d105ab6b8d
Merge "Move xenapi/agent.py to new call_xenapi style"