hostdev: Introduce virDomainHostdevSubsysSCSIiSCSI
    
    Create the structures and API's to hold and manage the iSCSI host device.
    This extends the 'scsi_host' definitions added in commit id '5c811dce'.
    A future patch will add the XML parsing, but that code requires some
    infrastructure to be in place first in order to handle the differences
    between a 'scsi_host' and an 'iSCSI host' device.