Treat instance like an object in _default_block_device_names

The _default_block_device_names method already gets an instance object
but was treating it like a primitive dict so this change enforces the
object usage with dot notation when accessing fields.

Also updates the existing unit tests to use an instance object.

Part of blueprint compute-manager-objects-juno

Change-Id: I2706fa081ba427956dc4f643d1e5664eaa2573ed
Merge "Treat instance like an object in _default_block_device_names"