* src/driver.h: add an extra entry point in the structure
* src/esx/esx_driver.c src/lxc/lxc_driver.c src/opennebula/one_driver.c
  src/openvz/openvz_driver.c src/phyp/phyp_driver.c src/qemu/qemu_driver.c
  src/remote/remote_driver.c src/test/test_driver.c src/uml/uml_driver.c
  src/vbox/vbox_tmpl.c src/xen/xen_driver.c: add NULL entry points for
  all drivers