class PortWatcher

Get the next port event.

The first N calls return [`DevicePortEvent.existing`] where N is the

number of ports present on the device at the time of the watcher's

creation. The next call returns [`DevicePortEvent.idle`] to indicate the

end of existing ports. Subsequent calls block until a port is added

([`DevicePortEvent.added`]) or removed ([`DevicePortEvent.removed`]).

The server closes the `PortWatcher` channel with `ZX_ERR_CANCELED` if

the number of unread events reaches a server-selected limit that is at

least two times [`MAX_PORTS`]. Clients are encouraged to maintain a

hanging call to `Watch` at all times to avoid triggering this condition.

- response `event` next port event.