roles: bifrost-create-vm-nodes: Set CPU model to 'host-model'

The default CPU model is the QEMU GenuineIntel/QEMU Virtual CPU which
doesn't offer the best possible performance. It's best to switch to the
'host-model' option which should make the vCPU more similar to the
host one.

Change-Id: I677064d14f53eaf6757c07016f4c22909f1b240b

playbooks/roles/bifrost-create-vm-nodes/defaults/main.yml

@@ -5,6 +5,7 @@ test_vm_memory_size: "3072"
 test_vm_num_nodes: 1
 test_vm_domain_type: "qemu"
 test_vm_arch: "x86_64"
+test_vm_cpu: "host-model"
 test_vm_groups: {}
 test_vm_default_groups: "{{ lookup('env', 'DEFAULT_HOST_GROUPS').split() | default(['baremetal'], true) }}"
 test_vm_disk_gib: "{{ lookup('env', 'VM_DISK') | default(10, true) }}"

playbooks/roles/bifrost-create-vm-nodes/templates/testvm.xml.j2

@@ -13,6 +13,9 @@
     <apic/>
     <pae/>
   </features>
+  <cpu mode='{{ test_vm_cpu }}'>
+    <model fallback='allow'/>
+  </cpu>
   <clock offset='utc'/>
   <on_poweroff>destroy</on_poweroff>
   <on_reboot>restart</on_reboot>

releasenotes/notes/vm-host-cpu-type-support-a09768f360372506.yaml

@@ -0,0 +1,7 @@
+---
+features:
+  - Add support for tweaking the vCPU model for the VMs created
+    by bifrost-create-vm-nodes. The default vCPU model is 'host-model'
+    which should provide the best possible performance whilst using only
+    the CPU features which are understood by libvirt. The model can be
+    changed using the `test_vm_cpu` Ansible variable.