tools/virtio: virtio_test -h,--help should return directly

When we get help information, we should return directly, and we should not
execute test cases. Move the exit() directly into the help() function and
remove it from case '?'.

Signed-off-by: Rong Tao <rongtao@cestc.cn>
Message-Id: <tencent_822CEBEB925205EA1573541CD1C2604F4805@qq.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>