scsi: storvsc: use in place iterator function

In 4.12-rc1, new functions were added to support iterating over elements
in the vmbus event ring. This patch uses them to simplify the ring
buffer handling in virtual SCSI driver as well.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>