vhost: fix zero-copy server mode

This patch fixes the situation where vhost-user cannot start as server
with dequeue_zero_copy enabled.

Using flag instead of vsocket->is_server to determine whether vhost-user
is in client mode. Because vsocket->is_server is not ready at this time.

Fixes: 715070ea10e6 ("vhost: prevent zero-copy with incompatible client mode")
Cc: stable@dpdk.org
Signed-off-by: Xuan Ding <xuan.ding@intel.com>
Acked-by: Xiaolong Ye <xiaolong.ye@intel.com>
Tested-by: Yinan Wang <yinan.wang@intel.com>