Rename instance_type to flavor in xenapi virt driver

Renames 'instance_type' to 'flavor' in the xenapi virt driver and
associated tests.

Part of blueprint flavor-instance-type-dedup

Change-Id: I0f1a17b725102eab30ce90714b50444d8d140da0
Merge "Rename instance_type to flavor in xenapi virt driver"