hostdev: Introduce virDomainHostdevSubsysSCSIiSCSI

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.