virtio-scsi: Batched prepare for cmd reqs

Queue the popped requests while calling
virtio_scsi_handle_cmd_req_prepare(), then submit them after all
prepared.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/hw/virtio/virtio-scsi.h

@@ -213,6 +213,10 @@ typedef struct VirtIOSCSIReq {
     VirtQueueElement elem;
     /* Set by dataplane code. */
     VirtIOSCSIVring *vring;
+
+    /* Used for two-stage request submission */
+    QTAILQ_ENTRY(VirtIOSCSIReq) next;
+
     SCSIRequest *sreq;
     size_t resp_size;
     enum SCSIXferMode mode;