vhost/scsi: Check LUN structure byte 0 is set to 1, per spec

The virtio spec requires byte 0 of the virtio-scsi LUN structure
to be '1'.

Signed-off-by: Venkatesh Srinivas <venkateshs@google.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>