[SCSI] vmw_pvscsi: Add support for I/O requests coalescing.

This change allows pvscsi driver to coalesce I/O requests
before issuing them. The number of I/O's coalesced can be
dynamically configured based on the workload.

Signed-off-by: Rishi Mehta <rmehta@vmware.com>
Signed-off-by: Arvind Kumar <arvindkumar@vmware.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>