Workaround needed to run vPTX on an AMD CPU

We have to ask libvirt to use 'svm' instead of 'vmx' in box-building
VM definition and Vagrantfile

Author: ipspace

docs/caveats.md

@@ -329,7 +329,8 @@ You can change the license file parameters within a node definition or with **de
 (caveats-vptx)=
 ## Juniper vPTX
 
-* *netlab* release 1.7.0 supports only vJunosEvolved releases that do not require external PFE- and RPIO links. The first vJunosEvolved release implementing internal PFE- and RPIO links is the release 23.2R1-S1.8.
+* *netlab* supports only vJunosEvolved releases that do not require external PFE- and RPIO links. The first vJunosEvolved release implementing internal PFE- and RPIO links is the release 23.2R1-S1.8.
+* vJunosEvolved software is supposed to be run on an Intel CPU. _netlab_ implements the hacks [suggested by Juniper to run vJunos on an AMD CPU](https://www.juniper.net/documentation/us/en/software/nce/nce-510-virtual-switches-mist-cloud-managed/amd-cpu_unofficial_tweaks.html), but please note that this is not supported.
 * The virtual MAC address of the anycast gateway is ignored. _netlab_, therefore, does not support the anycast gateway on vPTX.
 
 The rest of this section lists information you might find helpful if you're a long-time Junos user:

netsim/install/libvirt/vptx.xml.j2

@@ -19,10 +19,14 @@
   </features>
   <cpu>
      <arch>x86_64</arch>
+{% if 'amd' in cpu %}
+    <feature policy='require' name='svm'/>
+{% else %}
      <model>IvyBridge</model>
+     <feature name="vmx" policy="require"/>
+{% endif %}
      <topology cores="4" sockets="1" threads="1"/>
      <model fallback="allow">qemu64</model>
-     <feature name="vmx" policy="require"/>
   </cpu>
   <clock offset="utc"/>
   <on_poweroff>destroy</on_poweroff>

netsim/templates/provider/libvirt/vptx-domain.j2

@@ -15,7 +15,11 @@
       domain.nested = true
       domain.cpu_mode = "custom"
       domain.cpu_model = "qemu64"
+{% if 'amd' in defaults.processor %}
+      domain.cpu_feature :name => 'svm', :policy => 'require'
+{% else %}
       domain.cpu_feature :name => 'vmx', :policy => 'require'
+{% endif %}
       domain.sysinfo = {
         "bios": {
           "vendor": "Bochs",