[ddk] Tighten up access to device_publish_metadata()

This function is now restricted to drivers adding metadata to themselves
or their child devices, with the exception of driver running in the "sys" devhost,
which may publish metadata to arbitrary paths.

TEST: added metadata tests to ddk-test

Change-Id: I7f542dda648b738008766f5b2e7d2ea846197689

[ddk][test] Clean up ddk tests

Moved usb-request test to system/dev/test/ddk-test.
We can use this as a central location for adding new tests for DDK APIs.

Removed obsolete iotxn-test.

TEST: runtests in qemu, verified that driver-tests run usb-request and ddktl tests successfully

Change-Id: I63bbef51660c0af037958888567e13ae10afc9fc