vhost: raise error on fdset-thread creation

vhost: raise error on fdset-thread creation

When first call the 'rte_vhost_driver_start', the
fdset_event_dispatch thread should be created successfully.
Because the vhost uses it to poll socket events for vhost
server or clients. Without it, for example, vhost will not
get the connection event.

This patch returns err code directly when created not successful.

Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Reviewed-by: Jianfeng Tan <jianfeng.tan@intel.com>