Add some minimal implementation of virtio peripherals. At the level
of on-target hypervisor code we implement the MMIO layout and
maintain virtqueues. Once a buffer is available, we break into the
host proxyclient to deal with it.
Specific device-classes of the virtio spec ought to be implemented in
the proxyclient. Here the one device implemented is 9P transport,
exporting the m1n1 source directory.
Signed-off-by: Martin Povišer <povik@cutebit.org>
