xenvirt: fix reference to old mbuf field

Since commit 08b563ffb1 ("mbuf: replace data pointer by an offset"),
data is not an mbuf field anymore.

Signed-off-by: Sergio Gonzalez Monroy <sergio.gonzalez.monroy@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>

lib/librte_pmd_xenvirt/virtqueue.h

@@ -54,7 +54,7 @@
  * rather than gpa<->hva in virito spec.
  */
 #define RTE_MBUF_DATA_DMA_ADDR(mb) \
-	((uint64_t)((mb)->data))
+	rte_pktmbuf_mtod(mb, uint64_t)
 
 enum { VTNET_RQ = 0, VTNET_TQ = 1, VTNET_CQ = 2 };