[SCSI] virtio-scsi: initialize scatterlist structure

The sg struct is used without being initialized, which breaks
when CONFIG_DEBUG_SG is enabled.

Cc: stable@vger.kernel.org
Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>