conf: Use existing SCSI hostdev model to create new
    
    In virDomainDefMaybeAddHostdevSCSIcontroller when we add a new
    controller because someone neglected to add one or we're adding
    one because the existing one is full, we should copy over the
    model number from the existing controller since whatever we
    create should at least have the same characteristics as the one
    we cannot use because it's full.
    
    NB: This affects the existing hostdev-scsi-autogen-address test
    which would add a default ('lsi') SCSI controller for the various
    scsi_host's that would create a controller for the hostdev.