[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

[uapp][xdc-test] Move xdc init code to new file.

The test hostapp will be exactly the same as the current device app,
except the init code. We will build the hostapp with a different
implementation of configure_xdc (to be added in future CL).

TEST= plug in debugging cable and run fx shell xdc-test -f file_name

Change-Id: Ide0175849435c4779c95e0bb1d8b412e472bb6ba

[uapp][xdc-test] Simple test app for writing a file via the XHCI debug
capability, from debug device to host.

Will add reading in a future CL.

Change-Id: I8acc91c903677a58867692cd6ae37086e69acd32