[SCSI] add scsi_host and scsi_target to scsi_bus

This patch implements scsi_host and scsi_target device types
and adds both to the scsi_bus.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>