Change example Makefiles to use snippets

[Ivan-Velickovic]

Both sDDF and the library part of libvmm contain Makefile snippets, our examples that have Makefile should call into these instead of duplicating things.

[Ivan-Velickovic]

Closed in:

• Change examples/simple to use new Makefile snippet system #77
• Change examples/virtio to use new Makefile snippet system #79
• Change examples/virtio-snd to use new Makefile snippet system #84

thanks to @alexandermbrown.