xen: initialize port::_sem
xen: initialize port::_sem

Now that the semaphore default constructor is gone, we must initialize
_sem.

syuu1228/seastar,cloudius-systems/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar,dreamsxin/seastar,dreamsxin/seastar,cloudius-systems/seastar,syuu1228/seastar,avikivity/seastar,avikivity/seastar,scylladb/seastar,cloudius-systems/seastar,dreamsxin/seastar,scylladb/seastar