nova-next: Start testing the q35 machine type

With the introduction of the libvirt-default-machine-type blueprint
operators are now able to change the default machine type used by an
existing environment more easily. This should hopefully allow more
deployments to switch to the modern q35 machine type, away from the
legacy pc machine type that is currently the default.

With this in mind this change aims to start testing the q35 machine type
as part of the nova-next job before we eventually change the default in
the future.

The following changes are made to the job to allow it to pass with q35:

- [libvirt]num_pcie_ports is increased to 24

This is required as more devices are now connected by default over PCIe.

- [compute-feature-enabled]ide_bus is disabled

The q35 machine type does not support an IDE bus so any tests using this
bus are now skipped.

Depends-On: https://review.opendev.org/c/openstack/devstack/+/779179
Depends-On: https://review.opendev.org/c/openstack/tempest/+/775630
blueprint: libvirt-default-machine-type
Change-Id: Iad1adbc23b31dd54a96299e7a8a4b622c15eed8d

.zuul.yaml

@@ -181,6 +181,7 @@
       volume multi-attach testing.
       Starting in Train, the job enabled counting quota usage from placement.
       Starting in Ussuri, the job was changed to multinode.
+      Starting in Wallaby, the job defaults to the q35 machine type.
       Runs all tempest compute API and most scenario tests concurrently.
     irrelevant-files: *dsvm-irrelevant-files
     # Run post-tempest tests like for nova-manage commands.
@@ -200,6 +201,11 @@
       devstack_local_conf:
         post-config:
           $NOVA_CPU_CONF:
+            libvirt:
+              # Increase the number of PCIe ports per instance given the q35
+              # machine type attaches more devices by default than pc
+              num_pcie_ports: 24
+              hw_machine_type: "x86_64=q35"
             compute:
               # Switch off the provider association refresh, which should
               # reduce the number of placement calls in steady state. Added in
@@ -221,6 +227,9 @@
           $TEMPEST_CONFIG:
             network-feature-enabled:
               qos_placement_physnet: public
+            compute-feature-enabled:
+              # The q35 machine type doesn't support an IDE bus
+              ide_bus: False
       devstack_localrc:
         # Added in Pike.
         NOVA_USE_SERVICE_TOKEN: True