net: add qemu_{configure,create}_nic_device(), qemu_find_nic_info()

net: add qemu_{configure,create}_nic_device(), qemu_find_nic_info()

Most code which directly accesses nd_table[] and nb_nics uses them for
one of two things. Either "I have created a NIC device and I'd like a
configuration for it", or "I will create a NIC device *if* there is a
configuration for it".  With some variants on the theme around whether
they actually *check* if the model specified in the configuration is
the right one.

Provide functions which perform both of those, allowing platforms to
be a little more consistent and as a step towards making nd_table[]
and nb_nics private to the net code.

One might argue that platforms ought to be consistent about whether
they create the unconfigured devices or not, but making significant
user-visible changes is explicitly *not* the intent right now.

The new functions leave the 'model' field of the NICInfo as NULL after
using it for the default NIC model, unlike the qemu_check_nic_model()
function which does set nd->model to match default_model explicitly.
This is acceptable because there is no code which consumes nd->model
except this NIC-matching code in net/net.c, and no reasonable excuse
for any code wanting to use nd->model in future.

Also export the qemu_find_nic_info() helper, as some platforms have
special cases they need to handle.

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