scsi: storvsc: set virt_boundary_mask in the scsi host template

This ensures all proper DMA layer handling is taken care of by the SCSI
midlayer.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>