virtio-scsi.h: Add virtio_scsi_cmd_req_pi + VIRTIO_SCSI_F_T10_PI bits

This patch adds a virtio_scsi_cmd_req_pi header as recommened by
Paolo that contains pi_bytesout + pi_bytesin elements used for
signaling when protection information buffers (in bytes) are
expected to preceed the data payload buffers.

Also add new VIRTIO_SCSI_F_T10_PI feature bit to be used to signal
host support.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Sagi Grimberg <sagig@dev.mellanox.co.il>
Cc: H. Peter Anvin <hpa@zytor.com>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>