Merge virtIO and virtIO PCI examples into one?

[Ivan-Velickovic]

With #173 we now have virtIO PCI devices working, an example system that uses them. I'm wondering, to make it a bit easier to maintain, whether we instead have a configuration flag to switch between the two kinds of backends when building the main virtIO example.

[Ivan-Velickovic]

I think I've got a better idea, a UEFI example that uses virtIO PCI on all architectures while the normal virtIO example uses PCI on x86-64 and MMIO on other architectures.