[ulib][xdc-host] Lib for connecting to xdc host.

The lib function GetStream registers the stream id and
waits for the server response.

Also adds a simple test host app that registers for a stream id.
This is built from the existing xdc-test code used for the device
side app, but with the host version of configure_xdc.

TEST= run out/build-zircon/tools/xdc-server and
out/build-zircon/tools/xdc-test-host -u -f file

Change-Id: Ic65ccf97edf660ca41e81891ffd0e3c406b01b43