libvirt: Refactor ISCSIDriver to support other iSCSI transports besides TCP

Currently open-iscsi in openstack does not allow the use of offload iscsi transports (cxgb4i, qla4xx, bnx2i etc) or other software iscsi transports (like iser). This blueprint aims to add support for this

On the initiator side the only difference between TCP and other transports is in the interface flag (--interface=[iface])
e.g. "iscsiadm -m discoverydb -t st -p ip:port -I cxgb4i --discover"

The required changes are:
1. Add -I parameter with a default value of "default" (Default implies iscsi_tcp transport, which is the same as no transport being given), and have a set of allowed values (as defined by iscsiadm)