xenvirt: fix 32-bit build

xenvirt: fix 32-bit build

Compilation fails on 32 bits on Xen driver, due to wrong casting:

drivers/net/xenvirt/virtqueue.h: In function ‘virtqueue_enqueue_xmit’:
drivers/net/xenvirt/virtqueue.h:234:24: error: cast from pointer to integer
                            of different size [-Werror=pointer-to-int-cast]
  start_dp[idx].addr  = rte_pktmbuf_mtod(cookie, uint64_t);
                        ^

Fixes: d6b324c00fc9 ("mbuf: get DMA address")
Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>