New APIs are added allowing streaming of content to/from
storage volumes.

* include/libvirt/libvirt.h.in: Add virStorageVolUpload and
  virStorageVolDownload APIs
* src/driver.h, src/libvirt.c, src/libvirt_public.syms: Stub
  code for new APIs
* src/storage/storage_driver.c, src/esx/esx_storage_driver.c:
  Add dummy entries in driver table for new APIs