hw/arm: xenpvh: Reverse virtio-mmio creation order

hw/arm: xenpvh: Reverse virtio-mmio creation order

We've been creating the virtio-mmio devices in forwards order
but since the qbus lists prepend (rather than append) entries,
the virtio busses end up with decreasing base address order.

Xen enables virtio-mmio nodes in forwards order so there's been
a missmatch. So far, we've been working around this with an
out-of-tree patch to Xen.

This reverses the order making sure the virtio busses end up
ordered with increasing base addresses avoiding the need to
patch Xen.

Signed-off-by: Edgar E. Iglesias <edgar.iglesias@amd.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>