scsi-disk: add some optional scsi commands

I use a legacy OS which depends on some optional SCSI commands.
In fact this implementation does nothing special, but provides minimum
support for the following commands:

REZERO UNIT
WRITE AND VERIFY(10)
WRITE AND VERIFY(12)
WRITE AND VERIFY(16)
MODE SELECT(6)
MODE SELECT(10)
SEEK(6)
SEEK(10)

Signed-off-by: Bernhard Kohl <bernhard.kohl@nsn.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
