Add iSCSI backend storage driver for ESX

Add iSCSI backend storage driver for ESX

The patch adds the backend driver to support iSCSI format storage pools
and volumes for ESX host. The mapping of ESX iSCSI specifics to Libvirt
is as follows:

1. ESX static iSCSI target <------> Libvirt Storage Pools
2. ESX iSCSI LUNs          <------> Libvirt Storage Volumes.

The above understanding is based on http://libvirt.org/storage.html.

The operation supported on iSCSI pools includes:

1. List storage pools & volumes.
2. Get XML descriptor operaion on pools & volumes.
3. Lookup operation on pools & volumes by name, UUID and path (if applicable).

iSCSI pools does not support operations such as: Create / remove pools
and volumes.