examples/vhost: fix strict aliasing

examples/vhost: fix strict aliasing

Fixes following error on gcc 4.4.7:

examples/vhost/main.c: In function ‘new_device’:
rte_ring.h:740: error:
    dereferencing pointer ‘mbuf.486’ does break strict-aliasing rules
examples/vhost/main.c:1503: note: initialized from here
...
rte_ring.h:740: error:
    dereferencing pointer ‘({anonymous})’ does break strict-aliasing rules
examples/vhost/main.c:1804: note: initialized from here

Fixes: d19533e8 ("examples/vhost: copy old vhost example")
Reported-by: Qian Xu <qian.q.xu@intel.com>
Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Huawei Xie <huawei.xie@intel.com>