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 launch emulated Xen guests with '-nic user', and this 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>