hw/i386/pc: support '-nic' for xen-net-device

The default NIC creation seems a bit hackish to me. I don't understand
why each platform has to call pci_nic_init_nofail() from a point in the
code where it actually has a pointer to the PCI bus, and then we have
the special cases for things like ne2k_isa.

If qmp_device_add() can *find* the appropriate bus and instantiate
the device on it, why can't we just do that from generic code for
creating the default NICs too?

But that isn't a yak I want to shave today. Add a xenbus field to the
PCMachineState so that it can make its way from pc_basic_device_init()
to pc_nic_init() and be handled as a special case like ne2k_isa is.

Now we can launch emulated Xen guests with '-nic user'.

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