Avoid performance bottleneck when being SCSI pass-through'd to
virtual machines with other OSes (e.g. Windows) via virtio-scsi
and scsi-block in qemu.

Ref.: https://github.com/YanVugenfirer/kvm-guest-drivers-windows/issues/63Signed-off-by: Tom Yan <tom.ty89@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>