vhost: fix race for connection fd

vhost: fix race for connection fd

In the event handler of connection fd, the connection fd could be possibly
closed. The event dispatch loop would then try to remove the fd from fdset.
Between these two actions, another thread might register a new listenfd
reusing the val of just closed fd, so we couldn't call fdset_del which would
wrongly clean up the new listenfd. A new function fdset_del_slot is provided
to cleanup the fd at the specified location.

Signed-off-by: Huawei Xie <huawei.xie@intel.com>