Change the interface between XendRoot.get_network_script and Vifctl so that a

Change the interface between XendRoot.get_network_script and Vifctl so that a
list is passed rather than a single string.  This expands the config-file
interface so that parameters may be passed to the network scripts from the
config file.

Fix XendRoot._logError (missing asterisk before args parameter).

In XendRoot, allow an empty configuration file.  This makes it easier to unit
test code reliant on XendRoot.

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