kvm: Protect against buildout-unfriendly values

Even if boot-image-url-select is not designed to contain bad values, they can
happen, so add protection just like with boot-image-url-list.

software/kvm/buildout.hash.cfg

@@ -19,7 +19,7 @@ md5sum = e6d5c7bb627b4f1d3e7c99721b7c58fe
 
 [template-kvm]
 filename = instance-kvm.cfg.jinja2
-md5sum = 5e41dfce45f7be1c5e849cc7ffcc555b
+md5sum = 89796d6d6a25f694291f9b45181830ad
 
 [template-kvm-cluster]
 filename = instance-kvm-cluster.cfg.jinja2.in

software/kvm/instance-kvm.cfg.jinja2

@@ -972,7 +972,7 @@ keyboard-layout-language = fr
 {% set key_list =  v.split('\n') -%}
 {{ k }} =
   {{ key_list | join('\n  ') }}
-{% elif k == 'boot-image-url-list' %}
+{% elif k in ['boot-image-url-list', 'boot-image-url-select'] %}
 {# needs to decorate possibly multiline or maybe unsafe value #}
 {{ k }} = {{ dumps(v) }}
 {% else -%}