This defines the internal driver API and stubs out each driver

* src/driver.h: Define virDrvDomainGetBlockInfo signature
* src/libvirt.c, src/libvirt_public.syms: Glue public API to drivers
* 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/test/test_driver.c, src/uml/uml_driver.c, src/vbox/vbox_tmpl.c,
  src/xen/xen_driver.c, src/xenapi/xenapi_driver.c: Stub out driver