vhost-scsi: Clarify vhost_virtqueue_mask argument

vhost-scsi: Clarify vhost_virtqueue_mask argument

vhost_virtqueue_mask takes an "absolute" virtqueue index, while the
code looks like it's passing an index that is relative to
s->dev.vq_index.  In reality, s->dev.vq_index is always zero, so
this patch does not make any difference, but the code is clearer.

Signed-off-by: Lu Lina <lina.lulina@huawei.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Message-Id: <1437978359-17960-1-git-send-email-arei.gonglei@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>