kvm: Wipe out device data once KVM is removed.

This requires the "prerm" plugin be enabled to be used in addition to devperm. 

I tested on production and it works fine.

/cc @tomo

/reviewed-on nexedi/slapos!687

software/kvm/buildout.hash.cfg

@@ -19,7 +19,7 @@ md5sum = a236b719aaac61ac342ada0ce569151a
 
 [template-kvm]
 filename = instance-kvm.cfg.jinja2
-md5sum = 891d98667feb8daaa9f61b5bf09d5de0
+md5sum = d604d8696815716ac51af770164e86d5
 
 [template-kvm-cluster]
 filename = instance-kvm-cluster.cfg.jinja2.in

software/kvm/instance-kvm.cfg.jinja2

@@ -823,6 +823,14 @@ template = inline:
 rendered = ${buildout:directory}/.slapos-disk-permission
 context =
   raw disk_device_path {{disk_device_path}}
+
+{%   do part_list.append('wipe-disk-device-wrapper') -%}
+[wipe-disk-device-wrapper]
+recipe = slapos.cookbook:wrapper
+wrapper-path = ${directory:prerm}/slapos_wipe_device_disk
+command-line =
+  dd if=/dev/zero of={{disk_device_path}} bs=4096 count=500k
+
 {% endif -%}
 
 [buildout]