tests/libqos: virtio-mmio driver and interface nodes

tests/libqos: virtio-mmio driver and interface nodes

Add virtio-mmio node in qgraph framework.
virtio-mmio produces virtio-bus, the interface consumed by all virtio-*-device
nodes.

Being a memory-mapped device, it doesn't have to provide a constructor
to qgraph, since it's always "contained" inside some other nodes.

Signed-off-by: Emanuele Giuseppe Esposito <e.emanuelegiuseppe@gmail.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>