Implement a multiplexed VendorPassthru example

Change the Fake driver to use multiple VendorPassthru interfaces
to demonstrate to third-party driver authors one potential approach
to multiplexing multiple drivers together.

This is particularly useful when combining different power drivers
with the reference PXE driver, which has a component that must be
routed through pxe.VendorPassthru

Change-Id: I321c95454f8169a4990127af5bc94b602bc55aff
Merge "Implement a multiplexed VendorPassthru example"