This forum post is now out-of-date. The new Alpha versions published today include an updated version of Xamarin for Visual Studio that uses a new SSH-based connection to the Mac build host. This new connection no longer requires the old "Xamarin.iOS Build Host" app on the Mac.
This early Alpha release does not yet include a XamarinVS build. If you update the Mac build host, it will no longer be compatible with the current XamarinVS 3.11 releases. If you have already upgraded the Mac build host, you can downgrade back to the current Stable or Beta versions by switching updater channels.
This unusual situation has arisen because the Xamarin engineers are looking to get as much feedback as possible from users as soon as possible. For the moment, this early testing will be limited to Mac-only users, but XamarinVS builds will be made available when they are ready.
EDIT Sep 02: Update for new Alpha builds that resolve the issue.