fix uninitialized variable error in do_poll()

fix uninitialized variable error in do_poll()

Now that CONFIG_HVM can (and should) be turned off for the shim, gcc 8.2
apparently is no longer sure that "port" is indeed initialized at

    if ( sched_poll->nr_ports == 1 )
        v->poll_evtchn = port;

It doesn't look to be impossible for the compiler to prove it is not,
but we also can't rely on that to be the case. Add an initializer.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>