Remove libvirt_test.la library

Remove libvirt_test.la library

The libvirt_test.la library was introduced to allow test suites
to reference internal-only symbols. These days, nearly every
symbol we care about is in src/libvirt_private.syms, so there
is no need for libvirt_test.la to continue to exist

* src/Makefile.am: Delete libvirt_test.la & add new .syms files
* src/libvirt_private.syms: Export symbols needed by test suite
* tests/Makefile.am: Link to libvirt_test.la. Ensure LXC tests link
  to network_driver.la
* src/libvirt_esx.syms, src/libvirt_openvz.syms: Add exports needed
  by test suite