xen, vscsi: >2TB patch

xen, vscsi: >2TB patch

This patch fixes current 2TB limitation of the xen vscsi driver.
Both frontend (command size) and backend (missing READ_16/WRITE_16
commands) driver need a fix. Also an scsi command added to support
ALUA. Tested thoroughly on 4TB external RAID.

I also enabled some more commands to allow operating LTO tape
drives and autoloaders (tested on HP Ultrium 1/8 + Bacula).

Still more scsi commands might be missing for other specific
devices (why not to enable all scsi commands by default ?).

Signed-off-by: Samuel Kvasnica <samuel.kvasnica@ims.co.at>
Acked-by: Jan Beulich <jbeulich@novell.com>