virtio-blk: refuse SG_IO requests with scsi=off

QEMU does have a "scsi" option (to be used like -device
virtio-blk-pci,drive=foo,scsi=off).  However, it only
masks the feature bit, and does not reject the command
if a malicious guest disregards the feature bits and
issues a request.

Without this patch, using scsi=off does not protect you
from CVE-2011-4127.

Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
