hw/xen: use qemu_create_nic_bus_devices() to instantiate Xen NICs

hw/xen: use qemu_create_nic_bus_devices() to instantiate Xen NICs

When instantiating XenBus itself, for each NIC which is configured with
either the model unspecified, or set to to "xen" or "xen-net-device",
create a corresponding xen-net-device for it.

Now we can revert the previous more hackish version which relied on the
platform code explicitly registering the NICs on its own XenBus, having
returned the BusState* from xen_bus_init() itself.

This also fixes the setup for Xen PV guests, which was previously broken
in various ways and never actually managed to peer with the netdev.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>