Rename instance_type to flavor in vmware virt driver tests

Renames 'instance_type' to 'flavor' in the vmware virt driver tests.
There were no occurrences of 'instance_type' in the virt driver itself,
so this patch only changes the tests.

Part of blueprint flavor-instance-type-dedup

Change-Id: I81ea543e0ab480e35cf5b7b59bd9ecb1cf361c6f
Merge "Rename instance_type to flavor in vmware virt driver tests"