Switch to Cortex-A72 cpu on AArch64

CPU model "max" fails to boot CirrOS and some other images when QEMU 7.0
is used.

Change-Id: Ie34a883ac7ae989d7f74c19b9c9ee9f08a82a35f

tests/templates/nova-compute-overrides.j2

 [libvirt]
 {% if ansible_architecture == 'aarch64' %}
 cpu_mode=custom
-cpu_model=max
+# cpu_model=max fails to boot multiple images with QEMU 7.0 and -M virt (works with "-M virt-6.2")
+cpu_model=cortex-a72
 {% endif %}