examples/vhost: fix driver unregistering

examples/vhost: fix driver unregistering

The following commit broke vhost sample when it runs in second time:
292959c71961acde0cda6e77e737bb0a4df1559c

It should call api to unregister vhost driver when sample exit/quit, then
the socket file will be removed(by calling unlink), and thus make vhost sample
work correctly in the second time startup.

Test report: http://dpdk.org/ml/archives/dev/2015-July/020896.html

Fixes: 292959c71961 ("vhost: cleanup unix socket")

Signed-off-by: Changchun Ouyang <changchun.ouyang@intel.com>
Acked-by: Huawei Xie <huawei.xie@intel.com>