Add instructions for dumping the memory of a VirtualBox guest.

wiki/src/contribute/release_process/test/erase_memory_on_shutdown.mdwn

@@ -7,6 +7,8 @@
 Pick one of those:
 
 * [[erase_memory_on_shutdown/qemu_pmemsave]] (pros: no initial setup)
+* [[erase_memory_on_shutdown/virtualbox_dumpguestcore.mdwn]]
+  (pros: no initial setup)
 * [[erase_memory_on_shutdown/pxedump]] (pros: works for bare metal,
   possible to `|grep` and avoid writting the huge dump file to disk;
   cons: initial setup)

wiki/src/contribute/release_process/test/erase_memory_on_shutdown/virtualbox_dumpguestcore.mdwn

+Run the following from the host:
+
+    vboxmanage debugvm "${VM_NAME}" dumpguestcore --filename tails.dump
+
+and that's it.
+
+VirtualBox' VM core is an ELF that contains [some headers and other
+state](http://www.virtualbox.org/manual/ch12.html#guestcoreformat) of
+the Tails guest in addition to the memory dump, but there's no
+reasonable possibility of finding the pattern as false positives there
+so there's no reason to bother cutting the headers away.