Rename instance_type to flavor in libvirt virt driver tests

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

This also removes the unused _create_instance_type method in
test_libvirt.

Note that instance_type_id and instance_type_swap are not changed since
those are part of the database model and would require a database
migration to change them.

Part of blueprint flavor-instance-type-dedup

Change-Id: Iec71509e38aed1341f37194fb94dd34b10ac2f54
Merge "Rename instance_type to flavor in libvirt virt driver tests"