[SCSI] virtio-scsi: split locking per vq

Keep a separate lock for each virtqueue.  While not particularly
important now, it prepares the code for when we will add support
for multiple request queues.  It is also more tidy as soon as
we introduce a separate lock for the sglist.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>