nova: unset cpu_model on aarch64

Without this, running DevStack on an `aarch64` environment will end
up in cpu_model set to "Nehalem" and cpu_mode set to "host-passthrough"
which does not work.

This patch drops that value under aarch64 environments.

Change-Id: I30be5a388dda5ccf08718670dbb14a28a4a8a8eb
diff --git a/lib/nova_plugins/hypervisor-libvirt b/lib/nova_plugins/hypervisor-libvirt
index c1cd132..6c6f4c4 100644
--- a/lib/nova_plugins/hypervisor-libvirt
+++ b/lib/nova_plugins/hypervisor-libvirt
@@ -56,6 +56,10 @@
     # arm64-specific configuration
     if is_arch "aarch64"; then
         iniset $NOVA_CONF libvirt cpu_mode "host-passthrough"
+        # NOTE(mnaser): We cannot have `cpu_model` set if the `cpu_mode` is
+        #               set to `host-passthrough`, or `nova-compute` refuses to
+        #               start.
+        inidelete $NOVA_CONF libvirt cpu_model
     fi
 
     if isset ENABLE_FILE_INJECTION; then