xen/scsiback: vscsi >2TB patch

xen/scsiback: 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 ?).

This patch was tested on opensuse xenified kernels 2.6.31-2.6.34.7
in paravirtualized domU.

Signed-off-by: Samuel Kvasnica <samuel.kvasnica@ims.co.at>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>