[SCSI] vmw_pvscsi: Some improvements in pvscsi driver.

This change is about the following:
(1) If the number of targets is 16+ then default ring_pages to 32.
(2) Change default queue depth (per device) to 254.
(3) Implement change_queue_depth function so that queue_depth per device can
    be changed at run time. Honors the request only if coming from sysfs.
(4) Clean up the info returned by modinfo.

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