[machina][guest] Add BUILD.gn files.

Add BUILD.gn files to enable building the guest app using GN from
garnet. The GN-built guest app is called 'garnet-guest' for now to
avoid a name collision with the guest binary built by zircon.

Once we migrate machina and guest sources to garnet and stop building
guest in zircon we can revert this name back to simply 'guest'.

ZX-1379 #comment

Change-Id: I51863fee473e7a498cd27d4a6fa54c98b42ee97d

[virtio] Create virtio ulib.

This way we can share common structure and constant definitions
between host and client side implementations.

Change-Id: I565b9252e38a6d11c2df61efda649cb6ca6e246a