scsi: core: add a host / host template field for the virt boundary

This allows drivers setting it up easily instead of branching out to block
layer calls in slave_alloc, and ensures the upgraded max_segment_size
setting gets picked up by the DMA layer.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Kashyap Desai &lt; <kashyap.desai@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>