[Libguestfs] [PATCH nbdkit 0/4] Multiple valgrind improvements and possible security fix.

I worked out why valgrind wasn't being applied to nbdkit when run by many of the tests (patches 1-2). Unfortunately I'm not able to make it actually fail tests when valgrind fails. Although the situation is marginally improved in that you can now manually examine the *.log files and find valgrind failures that way. Also adds valgrinding of the Python plugin (patch 3). Along the way I found that when we create a TLS session object we never free it, which is a bit of a problem (although easy to fix - patch 4). I'll need to backport this fix to every stable branch. It's not clear how exploitable this is -- it's my feeling that you'd need to open millions of TLS sessions which would take forever, and the result would only be a denial of service as nbdkit runs out of memory and crashes. Rich.