Deprecate the nics option from the xm create configuration. Instead, we use the

Deprecate the nics option from the xm create configuration.  Instead, we use the
entries in vif to guide the configuration.  This is much less confusing.

Closes bug #440.

Change the example configuration files to match.  Also change them to use the
XenSource OUI in the MAC addresses.  Also change xm-test to match.

Remove the obsolete ipaddr configuration entry, and the backend_mac vif config
option.

Remove the preprocess_vifs function, folding it into the configure_vifs
function, and creating a simple comma_sep_kv_to_dict helper.

Remove the configure_vfr method, as it is unused.

Signed-off-by: Ewan Mellor <ewan@xensource.com>