Only perform symbol check against libvirt.so

Only perform symbol check against libvirt.so

The 'check-symfile' test case was checking the contents of
libvirt.syms against libvirt.so + all of libvirt_driver_XXX.so
This was in fact bogus - libvirt.syms should only refer to
stuff in libvirt.so, but it had some symbols from the various
driver modules in it too. Now that libvirt.syms has been
fixed, the check-symfile test can be simplified to only
consider libvirt.so