Prevent crash from dlclose() of libvirt.so

Prevent crash from dlclose() of libvirt.so

When libvirt calls virInitialize it creates a thread local
for the virErrorPtr storage, and registers a callback to
cleanup memory when a thread exits. When libvirt is dlclose()d
or otherwise made non-resident, the callback function is
removed from memory, but the thread local may still exist
and if a thread later exists, it will invoke the callback
and SEGV. There may also be other thread locals with callbacks
pointing to libvirt code, so it is in general never safe to
unload libvirt.so from memory once initialized.

To allow dlclose() to succeed, but keep libvirt.so resident
in memory, link with '-z nodelete'. This issue was first
found with the libvirt CIM provider, but can potentially
hit many of the dynamic language bindings which all ultimately
involve dlopen() in some way, either on libvirt.so itself,
or on the glue code for the binding which in turns links
to libvirt

* configure.ac, src/Makefile.am: Ensure libvirt.so is linked
  with -z nodelete
* cfg.mk, .gitignore, tests/Makefile.am, tests/shunloadhelper.c,
  tests/shunloadtest.c: A test case to unload libvirt while
  a thread is still running.