libvirt: Add a config update workflow test for [libvirt]hw_machine_type
libvirt: Add a config update workflow test for [libvirt]hw_machine_type

The previous changes in this series have added small targeted
functional tests as parts of the workflow were introduced. Now that
these are all in place this change adds a larger test that merges these
different parts of the workflow together to ensure it works as expected.

blueprint: libvirt-default-machine-type
Change-Id: I72a03e74fae8704546013d978e9c27a2573b219b

openstack/nova,openstack/nova,mahak/nova,mahak/nova,mahak/nova,openstack/nova