Fix symbol exports & remove duplicated libvirt_util.la linkage

Fix symbol exports & remove duplicated libvirt_util.la linkage

The libvirt_util.la library was mistakenly linked into libvirtd
directly. Since libvirt_util.la is already linked to libvirt.so,
this resulted in libvirtd getting two copies of the code and
more critically 2 copies of static global variables.

Testing in turn exposed a issue with loadable modules. The
gnulib replacement functions are not exported to loadable
modules. Rather than trying to figure out the name sof all
gnulib functions & export them, just linkage all loadable
modules against libgnu.la statically.

* daemon/Makefile.am: Remove linkage of libvirt_util.la
  and libvirt_driver.la
* src/Makefile.am: Link driver modules against libgnu.la
* src/libvirt.c: Don't try to load modules which were
  compiled out
* src/libvirt_private.syms: Export all other internal
  symbols that are required  by drivers