Mirage split netfront

The base mirage repository should contain the minimum amount of code to boot a controllable kernel/process on xen/Unix. At the moment it also contains the xen netfront driver: we should remove this into a separate library which can be used both by Mirage and also standalone (e.g. for testing). The block driver has already been removed successfully.

Once netfront has been removed, there will only be a few minor dependencies on xenstore left in the base repo, which will also be fairly easily removed.