virtio-scsi: Pack struct virtio_scsi_{req_cmd,resp_cmd}

Device needs the exact size of these data structure. Prevent padding.

This fixes guest hang when booting seabios + tcm_vhost.

Signed-off-by: Asias He <asias@redhat.com>
(cherry picked from commit b44a7be17bdd270ea029a8e2ec0c2e80c6cd0444)
diff --git a/src/virtio-scsi.h b/src/virtio-scsi.h
index bbfbf30..96c3701 100644
--- a/src/virtio-scsi.h
+++ b/src/virtio-scsi.h
@@ -26,7 +26,7 @@
     u8 prio;
     u8 crn;
     char cdb[VIRTIO_SCSI_CDB_SIZE];
-};
+} __attribute__((packed));
 
 /* This is the first element of the "in" scatter-gather list. */
 struct virtio_scsi_resp_cmd {
@@ -36,7 +36,7 @@
     u8 status;
     u8 response;
     u8 sense[VIRTIO_SCSI_SENSE_SIZE];
-};
+} __attribute__((packed));
 
 #define VIRTIO_SCSI_S_OK            0